add `pthread_equal`

[joboet]

From its documentation:

The pthread_equal() function is necessary because thread IDs should be considered opaque: there is no portable way for applications to directly compare two pthread_t values.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

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[tgross35]

Can you change this to target main? Can cherry pick to stable after merge.

@rustbot label +stable-nominated

[bors]

:umbrella: The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

I don't know why freebsd would have failed, it should be available https://man.freebsd.org/cgi/man.cgi?query=pthread_equal&sektion=3.

edit: looks like maybe it was something spurious or just had the outdated CI job. Will try with this rebase.