[libc++] Remove non-atomic "platform" semaphore implementations.

These can't be made constexpr-constructible (constinit'able),
so they aren't C++20-conforming. Also, the platform versions are
going to be bigger than the atomic/futex version, so we'd have
the awkward situation that `semaphore<42>` could be bigger than
`semaphore<43>`, and that's just silly.

Differential Revision: https://reviews.llvm.org/D110110

(cherry picked from commit d0eaf753)

CHERRY-PICK NOTE BY @ldionne:
I added a release note mentioning the ABI break.