libstdc++: Fix call to __glibcxx_rwlock_init (PR 93244)

libstdc++: Fix call to __glibcxx_rwlock_init (PR 93244)

When the target doesn't define PTHREAD_RWLOCK_INITIALIZER we use a
wrapper around pthread_wrlock_init, but the wrapper only takes one
argument and we try to call it with two.

This went unnnoticed on most targets because they do define the
PTHREAD_RWLOCK_INITIALIZER macro, but it causes a bootstrap failure on
darwin8.

	PR libstdc++/93244
	* include/std/shared_mutex [!PTHREAD_RWLOCK_INITIALIZER]
	(__shared_mutex_pthread::__shared_mutex_pthread()): Remove incorrect
	second argument to __glibcxx_rwlock_init.
	* testsuite/30_threads/shared_timed_mutex/94069.cc: New test.