Regression in threads tests on FreeBSD

[ararslan]

#53402 added thread affinity functions jl_getaffinity and jl_setaffinity. The added test fails on FreeBSD:

Error in testset threads:
Test Failed at /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-58927d885a/share/julia/test/threads.jl:366
  Expression: jl_setaffinity(1, mask, cpumasksize) == 0
   Evaluated: -22 == 0

The return code there in the failing test is EINVAL, meaning an argument is invalid. We could just not run that test on FreeBSD, as is currently done with Windows, but ideally we'd be able to fix the test (or implementation, I guess) to support FreeBSD.