Switch sem_t from bionic atomics to stdatomic.h.

Bug: 17572887
Change-Id: If66851ba9b831cdd698b9f1303289bb14448bd03
diff --git a/libc/include/semaphore.h b/libc/include/semaphore.h
index 5827870..4ef13af 100644
--- a/libc/include/semaphore.h
+++ b/libc/include/semaphore.h
@@ -36,7 +36,7 @@
 struct timespec;
 
 typedef struct {
-  volatile unsigned int count;
+  unsigned int count;
 #ifdef __LP64__
   int __reserved[3];
 #endif