Switch pthread_cond_t to <stdatomic.h>.

Bug: 17574458
Change-Id: Ic7f79861df4fe751cfa6c6b20b71123cc31e7114
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 4281132..2cbcd87 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -73,7 +73,7 @@
 };
 
 typedef struct {
-  int volatile value;
+  unsigned int value;
 #ifdef __LP64__
   char __reserved[44];
 #endif