tsan: unify __cxa_guard_acquire and pthread_once implementations

Currently we effectively duplicate "once" logic for __cxa_guard_acquire
and pthread_once. Unify the implementations.

This is not a no-op change:
 - constants used for pthread_once are changed to match __cxa_guard_acquire
   (__cxa_guard_acquire constants are tied to ABI, but it does not seem
   to be the case for pthread_once)
 - pthread_once now also uses PotentiallyBlockingRegion annotations
 - __cxa_guard_acquire checks thr->in_ignored_lib to skip user synchronization
It's unclear if these 2 differences are intentional or a mere sloppy inconsistency.
Since all tests still pass, let's assume the latter.

Reviewed By: vitalybuka, melver

Differential Revision: https://reviews.llvm.org/D107359