Support num_cpus on FreeBSD

[RalfJung]

Almost all tests pass on FreeBSD now, with the exception of:

FAILURES:
    tests/pass/shims/available-parallelism-miri-num-cpus.rs
    tests/pass/shims/available-parallelism.rs (revision stack)
    tests/pass/shims/available-parallelism.rs (revision tree)

This is caused by a lack of cpuset_getaffinity. We already support a similar function sched_getaffinity -- I don't know how similar the APIs are, though.

[LorrensP-2158466]

@rustbot claim