reorganize pthread data structures and move the definitions to alltypes.h

this allows sys/types.h to provide the pthread types, as required by
POSIX. this design also facilitates forcing ABI-compatible sizes in
the arch-specific alltypes.h, while eliminating the need for
developers changing the internals of the pthread types to poke around
with arch-specific headers they may not be able to test.

src/thread/pthread_rwlock_rdlock.c

@@ -3,6 +3,6 @@
 int pthread_rwlock_rdlock(pthread_rwlock_t *rw)
 {
 	while (pthread_rwlock_tryrdlock(rw))
-		__wait(&rw->__wrlock, &rw->__waiters, 1, 0);
+		__wait(&rw->_rw_wrlock, &rw->_rw_waiters, 1, 0);
 	return 0;
 }