Switch pthread_mutex_t from bionic atomics to <stdatomic.h>.

Bug: 17574456
Change-Id: I5ce3d3dc07e804e9ce55c42920f47531b56e04de
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 4281132..8d053ae 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -43,7 +43,7 @@
 #endif
 
 typedef struct {
-  int volatile value;
+  int value;
 #ifdef __LP64__
   char __reserved[36];
 #endif