Check sem_clockwait availability at runtime on Linux #160368

Bug report

A Linux Python built against older libc headers doesn't use sem_clockwait, even when the runtime libc provides it. The configure-time result means that binary keeps using realtime deadlines through sem_timedwait on newer systems.

In python-build-standalone, we build against an older libc baseline for portability. I'd like to upstream the runtime check tracked in astral-sh/python-build-standalone#1176, so timed semaphore waits can use CLOCK_MONOTONIC when sem_clockwait is available.

A weak reference would let supported Linux builds use the function when present and fall back to sem_timedwait otherwise. ThreadSanitizer builds would keep their realtime path.

This extends python/cpython#85876, which added the configure-time check.

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-160368: Detect sem_clockwait at runtime on Linux #160371