libstdc++: Guard mutex and condvar with gthreads macro [PR103638]

libstdc++: Guard mutex and condvar with gthreads macro [PR103638]

A mutex and condition variable is used for timed waits on atomics if
there is no "platform wait" (e.g. futex) supported. But the use of those
types wasn't guarded by the _GLIBCXX_HAS_GTHREADS macro, causing errors
for --disable-threads builds. This fix allows <atomic> to work on
targets with futexes but no gthreads.

libstdc++-v3/ChangeLog:

	PR libstdc++/103638
	* include/bits/atomic_timed_wait.h: Check _GLIBCXX_HAS_GTHREADS
	before using std::mutex and std::__condvar.