Linking with core_bench fails in a bytecode-only switch

Hello,

I am unable to build a program that uses `core_bench` under OCaml 4.09.0+bytecode-only.

To reproduce the problem, the following instructions suffice:

```
git clone git@github.com:janestreet/core_bench.git
cd core_bench
git checkout v0.13.0
dune build @all
```

This produces the following output:

```
File "_none_", line 1:
Error: Error while linking /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/core.cma(Core__Linux_ext):
The external function `core_linux_timerfd_settime' is not available
ocamlc test/test_bench.exe (exit 2)
(cd _build/default && /home/fpottier/.opam/4.09.0+bytecode-only/bin/ocamlc -w @1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -o test/test_bench.exe -custom /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base_internalhash_types/base_internalhash_types.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base_internalhash_types /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/caml/caml.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib0/sexplib0.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/shadow_stdlib/shadow_stdlib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_compare/runtime-lib/ppx_compare_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_sexp_conv/runtime-lib/ppx_sexp_conv_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_hash/runtime-lib/ppx_hash_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_enumerate/runtime-lib/ppx_enumerate_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/fieldslib/fieldslib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_here/runtime-lib/ppx_here_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_assert/runtime-lib/ppx_assert_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_bench/runtime-lib/ppx_bench_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_inline_test/config/inline_test_config.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_inline_test/runtime-lib/ppx_inline_test_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/splittable_random/splittable_random.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_quickcheck/base_quickcheck.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/md5/md5_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/unix.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/bigarray.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/variantslib/variantslib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot/shape/bin_shape_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot/bin_prot.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/config/expect_test_config.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/common/expect_test_common.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/collector/expect_test_collector.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/collector /home/fpottier/.opam/4.09.0+bytecode-only/lib/typerep/typerep_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/stdio/stdio.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/jane-street-headers/jane_street_headers.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/time_now/time_now.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/time_now /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_module_timer/runtime/ppx_module_timer_runtime.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_bigstring/base_bigstring.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_bigstring /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/base_for_tests/base_for_tests.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/parsexp/parsexp.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib/sexplib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/core_kernel.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/threads/threads.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/error_checking_mutex/error_checking_mutex.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/error_checking_mutex /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/bounded_int_table/bounded_int_table.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/flags/flags.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/version_util/version_util.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/version_util /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib/unix/sexplib_unix.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/spawn/spawn.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/spawn /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/core.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core /home/fpottier/.opam/4.09.0+bytecode-only/lib/textutils/console/console.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/textutils/ascii_table/ascii_table.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/time_stamp_counter/time_stamp_counter.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/time_stamp_counter src/core_bench.cma test/.test_bench.eobjs/byte/allocation_tests.cmo test/.test_bench.eobjs/byte/array_tests.cmo test/.test_bench.eobjs/byte/basic_tests.cmo test/.test_bench.eobjs/byte/exception_tests.cmo test/.test_bench.eobjs/byte/gc_tests.cmo test/.test_bench.eobjs/byte/tsc.cmo test/.test_bench.eobjs/byte/tuple_tests.cmo test/.test_bench.eobjs/byte/test_bench.cmo)
/tmp/ccKRPh7s.o:(.data+0x2b78): undefined reference to `core_linux_timerfd_settime'
/tmp/ccKRPh7s.o:(.data+0x2b80): undefined reference to `core_linux_timerfd_gettime'
/tmp/ccKRPh7s.o:(.data+0x2b88): undefined reference to `core_linux_timerfd_create'
/tmp/ccKRPh7s.o:(.data+0x2bc0): undefined reference to `core_linux_timerfd_CLOCK_REALTIME'
/tmp/ccKRPh7s.o:(.data+0x2bc8): undefined reference to `core_linux_timerfd_CLOCK_MONOTONIC'
/tmp/ccKRPh7s.o:(.data+0x2bd0): undefined reference to `core_linux_timerfd_TFD_NONBLOCK'
/tmp/ccKRPh7s.o:(.data+0x2bd8): undefined reference to `core_linux_timerfd_TFD_CLOEXEC'
collect2: error: ld returned 1 exit status
File "_none_", line 1:
Error: Error while building custom runtime system
ocamlc test/mv_regr.exe (exit 2)
(cd _build/default && /home/fpottier/.opam/4.09.0+bytecode-only/bin/ocamlc -w @1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -o test/mv_regr.exe -custom /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base_internalhash_types/base_internalhash_types.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base_internalhash_types /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/caml/caml.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib0/sexplib0.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/shadow_stdlib/shadow_stdlib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/base.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_compare/runtime-lib/ppx_compare_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_sexp_conv/runtime-lib/ppx_sexp_conv_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_hash/runtime-lib/ppx_hash_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_enumerate/runtime-lib/ppx_enumerate_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/fieldslib/fieldslib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_here/runtime-lib/ppx_here_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_assert/runtime-lib/ppx_assert_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_bench/runtime-lib/ppx_bench_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_inline_test/config/inline_test_config.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_inline_test/runtime-lib/ppx_inline_test_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/splittable_random/splittable_random.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_quickcheck/base_quickcheck.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base/md5/md5_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/unix.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/bigarray.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/variantslib/variantslib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot/shape/bin_shape_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot/bin_prot.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/bin_prot /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/config/expect_test_config.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/common/expect_test_common.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/collector/expect_test_collector.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_expect/collector /home/fpottier/.opam/4.09.0+bytecode-only/lib/typerep/typerep_lib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/stdio/stdio.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/jane-street-headers/jane_street_headers.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/time_now/time_now.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/time_now /home/fpottier/.opam/4.09.0+bytecode-only/lib/ppx_module_timer/runtime/ppx_module_timer_runtime.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_bigstring/base_bigstring.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/base_bigstring /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/base_for_tests/base_for_tests.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/parsexp/parsexp.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib/sexplib.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/core_kernel.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml/threads/threads.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/ocaml /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/error_checking_mutex/error_checking_mutex.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/error_checking_mutex /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/bounded_int_table/bounded_int_table.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/flags/flags.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/version_util/version_util.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core_kernel/version_util /home/fpottier/.opam/4.09.0+bytecode-only/lib/sexplib/unix/sexplib_unix.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/spawn/spawn.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/spawn /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/core.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core /home/fpottier/.opam/4.09.0+bytecode-only/lib/textutils/console/console.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/textutils/ascii_table/ascii_table.cma /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/time_stamp_counter/time_stamp_counter.cma -I /home/fpottier/.opam/4.09.0+bytecode-only/lib/core/time_stamp_counter src/core_bench.cma test/.test_bench.eobjs/byte/mv_regr.cmo)
/tmp/ccfrpcyq.o:(.data+0x2b78): undefined reference to `core_linux_timerfd_settime'
/tmp/ccfrpcyq.o:(.data+0x2b80): undefined reference to `core_linux_timerfd_gettime'
/tmp/ccfrpcyq.o:(.data+0x2b88): undefined reference to `core_linux_timerfd_create'
/tmp/ccfrpcyq.o:(.data+0x2bc0): undefined reference to `core_linux_timerfd_CLOCK_REALTIME'
/tmp/ccfrpcyq.o:(.data+0x2bc8): undefined reference to `core_linux_timerfd_CLOCK_MONOTONIC'
/tmp/ccfrpcyq.o:(.data+0x2bd0): undefined reference to `core_linux_timerfd_TFD_NONBLOCK'
/tmp/ccfrpcyq.o:(.data+0x2bd8): undefined reference to `core_linux_timerfd_TFD_CLOEXEC'
collect2: error: ld returned 1 exit status
File "_none_", line 1:
Error: Error while building custom runtime system
```

I am running Ubuntu LTS 16.04, in case that matters.

Start by reproducing the issue with the documented git checkout and `dune build @all` under OCaml 4.09.0+bytecode-only. Trace how `core_linux_timerfd_*` is linked into the `core.cma` dependency and compare the failing custom bytecode link command. Done means the build completes without the undefined references.

Written by the indexing model from the issue text.