Reserve some space in sem_t for the future.

Reserve 12 more bytes in sem_t to give room for future implementation
improvements. This gets us to a 16 bytes sem_t. Glibc uses 32 bytes (16
actual use + 16 reserved), while OpenBSD has 16 bytes (out of which 4
are for padding).

Bug: 14587103
Bug: 12875898
Change-Id: Id835cc5abf874c651e6b5ad5b8f29c9d6ab08d5a

libc/include/semaphore.h

@@ -34,6 +34,9 @@ __BEGIN_DECLS
 
 typedef struct {
   volatile unsigned int count;
+#ifdef __LP64__
+  int __reserved[3];
+#endif
 } sem_t;
 
 #define SEM_FAILED NULL