libstdc++: Remove #error from <semaphore> implementation [PR 100179]

libstdc++: Remove #error from <semaphore> implementation [PR 100179]

This removes the #error from <bits/semaphore_base.h> for the case where
neither __atomic_semaphore nor __platform_semaphore is defined.

Also rename the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro to
_GLIBCXX_USE_POSIX_SEMAPHORE for consistency with the similar
_GLIBCXX_USE_CXX11_ABI macro that can be used to request an alternative
(ABI-changing) implementation.

libstdc++-v3/ChangeLog:

	PR libstdc++/100179
	* include/bits/semaphore_base.h: Remove #error.
	* include/std/semaphore: Do not define anything unless one of
	the implementations is available.