access to undefined reference in Distributed tests

From https://github.com/JuliaLang/julia/pull/39591:

tester_linux64:

ERROR: LoadError: On worker 23:
UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:33
 [2] JuliaLang/julia#447
   @ /buildworker/worker/tester_linux64/build/share/julia/stdlib/v1.7/Distributed/test/distributed_exec.jl:1607
 [3] JuliaLang/julia#106
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/process_messages.jl:278
 [4] run_work_thunk
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/process_messages.jl:63
 [5] macro expansion
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/process_messages.jl:278 [inlined]
 [6] JuliaLang/julia#105
   @ ./task.jl:406
Stacktrace:
 [1] (::Base.var"#876#878")(x::Task)
   @ Base ./asyncmap.jl:177
 [2] foreach(f::Base.var"#876#878", itr::Vector{Any})
   @ Base ./abstractarray.jl:2146
 [3] maptwice(wrapped_f::Function, chnl::Channel{Any}, worker_tasks::Vector{Any}, c::Vector{Int64})
   @ Base ./asyncmap.jl:177
 [4] wrap_n_exec_twice
   @ ./asyncmap.jl:153 [inlined]
 [5] async_usemap(f::var"#446#448", c::Vector{Int64}; ntasks::Int64, batch_size::Nothing)
   @ Base ./asyncmap.jl:103
 [6] #asyncmap#860
   @ ./asyncmap.jl:81 [inlined]
 [7] asyncmap
   @ ./asyncmap.jl:81 [inlined]
 [8] reuseport_tests()
   @ Main /buildworker/worker/tester_linux64/build/share/julia/stdlib/v1.7/Distributed/test/distributed_exec.jl:1601
 [9] top-level scope
   @ /buildworker/worker/tester_linux64/build/share/julia/stdlib/v1.7/Distributed/test/distributed_exec.jl:1637
in expression starting at /buildworker/worker/tester_linux64/build/share/julia/stdlib/v1.7/Distributed/test/distributed_exec.jl:1636

Log: https://build.julialang.org/#/builders/71/builds/566/steps/5/logs/stdio (uploaded rr trace: rr-run_566-gitsha_2da8c98287-2021-02-10_01_27_46.tar.zst)

Start with share/julia/stdlib/v1.7/Distributed/test/distributed_exec.jl around reuseport_tests at lines 1601-1637, then follow the stack into Distributed/src/process_messages.jl. Inspect the linked tester_linux64 log and uploaded rr trace to isolate the undefined reference; done means the reuseport test completes without this error.