Switch pthread_once_t to stdatomic.h.

Bug: 17574610

Change-Id: I653f2aa4b5b38dbdaffe4a6c3deccfe6ad3b0e74
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 2178789..4281132 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -112,7 +112,7 @@
 
 typedef int pthread_key_t;
 
-typedef volatile int pthread_once_t;
+typedef int pthread_once_t;
 
 #define PTHREAD_ONCE_INIT 0