ALLOW_BLOCKING_ON_MAIN_THREAD=0 works differently on Mac and Linux

Please include the following in your bug report:

**Version of emscripten/emsdk:**

```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.46-git
clang version 18.0.0 (https://github.com/llvm/llvm-project.git 75501f53624de92aafce2f1da698b249a7293dc7)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /opt/homebrew/Cellar/emscripten/3.1.46/libexec/llvm/bin
```

**Full link command and output with `-v` appended:**

```
emcc -Llib -pthread -s EXPORTED_RUNTIME_METHODS=["cwrap","stringToNewUTF8","UTF8ToString"] -s EXPORTED_FUNCTIONS=_free,_main,_malloc,_precache_file_data,_process_ucgi_command_wasm,_score_play,_destroy_cache,_ucgi_search_status_wasm,_ucgi_stop_search_wasm,_static_evaluation -sPTHREAD_POOL_SIZE="navigator.hardwareConcurrency + 3" -s ALLOW_BLOCKING_ON_MAIN_THREAD=0 -v obj/src/anchor.o obj/src/autoplay.o obj/src/bag.o obj/src/board.o obj/src/config.o obj/src/cross_set.o obj/src/fileproxy.o obj/src/game.o obj/src/game_event.o obj/src/game_history.o obj/src/gameplay.o obj/src/gcg.o obj/src/go_params.o obj/src/infer.o obj/src/klv.o obj/src/kwg.o obj/src/leave_map.o obj/src/leave_rack.o obj/src/letter_distribution.o obj/src/log.o obj/src/move.o obj/src/movegen.o obj/src/player.o obj/src/rack.o obj/src/random.o obj/src/sim.o obj/src/stats.o obj/src/string_util.o obj/src/thread_control.o obj/src/ucgi.o obj/src/ucgi_command.o obj/src/ucgi_formats.o obj/src/ucgi_print.o obj/src/util.o obj/src/wasm_api.o obj/src/winpct.o obj/src/words.o obj/src/xoshiro.o obj/wasmentry/api.o -lm -o bin/magpie_shell.html \
-s INITIAL_MEMORY=128MB \
--shell-file wasm_shell.html
"/opt/homebrew/Cellar/emscripten/3.1.46/libexec/llvm/bin/clang" --version
"/opt/homebrew/Cellar/emscripten/3.1.46/libexec/llvm/bin/wasm-ld" -o bin/magpie_shell.wasm -Llib obj/src/anchor.o obj/src/autoplay.o obj/src/bag.o obj/src/board.o obj/src/config.o obj/src/cross_set.o obj/src/fileproxy.o obj/src/game.o obj/src/game_event.o obj/src/game_history.o obj/src/gameplay.o obj/src/gcg.o obj/src/go_params.o obj/src/infer.o obj/src/klv.o obj/src/kwg.o obj/src/leave_map.o obj/src/leave_rack.o obj/src/letter_distribution.o obj/src/log.o obj/src/move.o obj/src/movegen.o obj/src/player.o obj/src/rack.o obj/src/random.o obj/src/sim.o obj/src/stats.o obj/src/string_util.o obj/src/thread_control.o obj/src/ucgi.o obj/src/ucgi_command.o obj/src/ucgi_formats.o obj/src/ucgi_print.o obj/src/util.o obj/src/wasm_api.o obj/src/winpct.o obj/src/words.o obj/src/xoshiro.o obj/wasmentry/api.o -L/opt/homebrew/Cellar/emscripten/3.1.46/libexec/cache/sysroot/lib/wasm32-emscripten /opt/homebrew/Cellar/emscripten/3.1.46/libexec/cache/sysroot/lib/wasm32-emscripten/crtbegin.o -lGL-mt -lal -lhtml5 -lbulkmemory -lstubs-debug -lnoexit -lc-mt-debug -ldlmalloc-mt -lcompiler_rt-mt -lc++-mt-noexcept -lc++abi-debug-mt-noexcept -lsockets-mt -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /var/folders/jy/zkj5g46j1dzf8304s7ybvny40000gn/T/tmp__30ovonlibemscripten_js_symbols.so --import-memory --shared-memory --strip-debug --export-if-defined=free --export-if-defined=main --export-if-defined=malloc --export-if-defined=precache_file_data --export-if-defined=process_ucgi_command_wasm --export-if-defined=score_play --export-if-defined=destroy_cache --export-if-defined=ucgi_search_status_wasm --export-if-defined=ucgi_stop_search_wasm --export-if-defined=static_evaluation --export-if-defined=_emscripten_thread_init --export-if-defined=_emscripten_thread_exit --export-if-defined=_emscripten_thread_crashed --export-if-defined=_emscripten_tls_init --export-if-defined=pthread_self --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export-if-defined=fflush --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=stackSave --export=stackRestore --export=stackAlloc --export=__errno_location --export=_emscripten_thread_free_data --export=emscripten_main_runtime_thread_id --export=emscripten_main_thread_process_queued_calls --export=_emscripten_run_on_main_thread_js --export=emscripten_stack_set_limits --export=__get_temp_ret --export=__set_temp_ret --export=__wasm_call_ctors --export=_emscripten_thread_init --export=_emscripten_thread_exit --export=malloc --export-table -z stack-size=65536 --initial-memory=134217728 --max-memory=134217728 --no-entry --stack-first --table-base=1
"/opt/homebrew/Cellar/emscripten/3.1.46/libexec/llvm/bin/llvm-objcopy" bin/magpie_shell.wasm bin/magpie_shell.wasm --remove-section=.debug* --remove-section=producers
"/opt/homebrew/Cellar/emscripten/3.1.46/libexec/binaryen/bin/wasm-emscripten-finalize" --dyncalls-i64 --pass-arg=legalize-js-interface-exported-helpers bin/magpie_shell.wasm -o bin/magpie_shell.wasm --detect-features
"/opt/homebrew/opt/node/bin/node" /opt/homebrew/Cellar/emscripten/3.1.46/libexec/src/compiler.js /var/folders/jy/zkj5g46j1dzf8304s7ybvny40000gn/T/tmpqm_4kdez.json
"/opt/homebrew/opt/node/bin/node" /opt/homebrew/Cellar/emscripten/3.1.46/libexec/tools/preprocessor.js /var/folders/jy/zkj5g46j1dzf8304s7ybvny40000gn/T/emscripten_temp_iou3le6h/settings.js worker.js --expandMacros
"/opt/homebrew/opt/node/bin/node" /opt/homebrew/Cellar/emscripten/3.1.46/libexec/tools/preprocessor.js /var/folders/jy/zkj5g46j1dzf8304s7ybvny40000gn/T/emscripten_temp_iou3le6h/settings.js wasm_shell.html
```

-------

I have a program where the main thread does not block; it exits immediately. When invoked from JS, a function creates a pthread and detaches immediately, thus not blocking. On Linux, the generated html shell works as designed. When compiled with the same version of emcc on Mac, with the same exact flags/code/Makefile/etc, the program prints "Blocking on the main thread is very dangerous" on the JS console and aborts.