Switch pthread_rwlock_t to stdatomic.

Bug: 19099838
Change-Id: Ie82967a60b5cec61a8bdd1e0e4a03738d01944f8
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 212551b..1fe61e2 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -91,10 +91,10 @@
   pthread_mutex_t __unused_lock;
   pthread_cond_t __unused_cond;
 #endif
-  volatile int32_t state; // 0=unlock, -1=writer lock, +n=reader lock
-  volatile int32_t writer_thread_id;
-  volatile int32_t pending_readers;
-  volatile int32_t pending_writers;
+  int32_t state; // 0=unlock, -1=writer lock, +n=reader lock
+  int32_t writer_thread_id;
+  uint32_t pending_readers;
+  uint32_t pending_writers;
   int32_t attr;
 #ifdef __LP64__
   char __reserved[36];