pthread-thread, lock: On z/OS, use PTHREAD_RWLOCK_INITIALIZER_NP.
Reported by Daniel Richard G. in
<https://lists.gnu.org/archive/html/bug-gnulib/2019-11/msg00001.html>
<https://lists.gnu.org/archive/html/bug-gnulib/2019-12/msg00167.html>
* lib/pthread.in.h (PTHREAD_RWLOCK_INITIALIZER): Define to
PTHREAD_RWLOCK_INITIALIZER_NP when possible.
* lib/glthread/lock.h: Allow PTHREAD_RWLOCK_INITIALIZER_NP as an
alternative to PTHREAD_RWLOCK_INITIALIZER.
* lib/glthread/lock.c: Likewise.