Add pthread_once

[0xcaff]

Close #3157

[rustbot]

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[0xcaff]

PR in draft while waiting for CI to complete. Maintainer, please feel free to take over and undraft it if the CI is green and I forgot.

[0xcaff]

This PR adds pthread_once to linux_like platforms (linux, android, l4re and emscripten). pthread_once seems to be more widely supported than on these linux_like platforms:

• apple's platforms (macOS, iOS, etc.) https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/pthread_once.3.html

• freebsd https://man.freebsd.org/cgi/man.cgi?query=pthread_once&sektion=3&apropos=0&manpath=FreeBSD+7.1-RELEASE

• openbsd https://man.openbsd.org/pthread_once.3

• musl https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_once.c

• opensolaris https://docs.oracle.com/cd/E19120-01/open.solaris/816-5137/tlib-43636/index.html

Given that this is pretty widely available and not a platform specific feature (pthread_once is part of POSIX), should pthread_once go into src/unix/mod.rs instead? I see pthread_cancel is similar (part of POSIX) but instead of it being defined in src/unix/mod.rs, it is defined in what seems like every platform's module.

I guess it comes down to how bad getting this wrong is. The tests only seem to cover a few platforms (macOS, Windows, Linux) so perhaps we should err on the side of safety by not adding symbols to targets we're not absolutely sure about?

[JohnTitor]

If you want to use the item on these platforms, feel free! Note that you don't have to declare an item on all the available platforms exhaustively. You could just move it whenever you need it.

[JohnTitor]

For now, I'm going to r+, but feel free to move the items if you want :)
@bors r+

[bors]

📌 Commit 7d40148 has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7d40148 with merge c6a07e6...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing c6a07e6 to main...