libstdc++: Unblock atomic wait on non-futex platforms [PR106183]

When using a mutex and condition variable, the notifying thread needs to
increment _M_ver while holding the mutex lock, and the waiting thread
needs to re-check after locking the mutex. This avoids a missed
notification as described in the PR.

By moving the increment of _M_ver to the base _M_notify we can make the
use of the mutex local to the use of the condition variable, and
simplify the code a little. We can use a relaxed store because the mutex
already provides sequential consistency. Also we don't need to check
whether __addr == &_M_ver because we know that's always true for
platforms that use a condition variable, and so we also know that we
always need to use notify_all() not notify_one().

Reviewed-by: Thomas Rodgers <trodgers@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/106183
	* include/bits/atomic_wait.h (__waiter_pool_base::_M_notify):
	Move increment of _M_ver here.
	[!_GLIBCXX_HAVE_PLATFORM_WAIT]: Lock mutex around increment.
	Use relaxed memory order and always notify all waiters.
	(__waiter_base::_M_do_wait) [!_GLIBCXX_HAVE_PLATFORM_WAIT]:
	Check value again after locking mutex.
	(__waiter_base::_M_notify): Remove increment of _M_ver.