FreeBSD: Add cpuset and physical core support

[Freaky]

This adds support for retrieving the physical processor count by querying the kern.smp.cores sysctl, and also uses cpuset_getaffinity to determine the logical thread count. Both will fall back to the original code on failure.

❯ cargo run --example values
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/examples/values`
Logical CPUs: 16
Physical CPUs: 8
❯ cpuset -l 0-1 cargo run --example values
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/examples/values`
Logical CPUs: 2
Physical CPUs: 8
❯ cpuset -l 4-7 cargo run --example values
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/examples/values`
Logical CPUs: 4
Physical CPUs: 8

Tested on 13.1-RELEASE-p7/amd64.

[seanmonstar]

Thanks for the PR! Does our current CI handle this? If not, can we add a job?

[seanmonstar]

Suggested change

	runs-on: macos-12
	runs-on: ubuntu-latest

Seems like the Ubuntu jobs are faster than mac (I assume there's a smaller quantity of them).

[Freaky]

Unfortunately VM Actions only supports macos-12, as nested virtualisation isn't available elsewhere (at least on the free tier).