libstdc++: Make atomic<T*>::wait() const [PR102994]

libstdc++: Make atomic<T*>::wait() const [PR102994]

This was an oversight in the original commit adding wait/notify
to atomic<T>.

libstdc++-v3/ChangeLog:

	PR libstdc++/102994
	* include/bits/atomic_base.h (__atomic_base<_PTp*>::wait()):
	Add const qualifier.
	* include/std/atomic (atomic<_Tp*>::wait(), atomic_wait()):
	Likewise.
	* testsuite/29_atomics/atomic/wait_notify/102994.cc:
	New test.