[libc++] Switch to the officially documented APIs for implementing ulock_{wait,wake} on Apple

[ldionne]

While discussing with @huixie90 and Apple folks about #109290, it was noticed that we were still using forward-declarations of __ulock_wait and __ulock_wake. Instead, the public API os_sync_wait_on_address is now provided and documented, so we should switch to that. This can be a simple NFC refactoring.