libc/pthread: Avoid compiler optimizations for once_control->done

Prevent compiler reordering that may read stale value of once_control->done.
Use volatile to ensure visibility across threads and prevent incorrect once initialization.

Signed-off-by: yukangzhi <yukangzhi@xiaomi.com>

include/pthread.h

@@ -402,7 +402,7 @@ typedef struct pthread_barrier_s pthread_barrier_t;
 
 struct pthread_once_s
 {
-  bool done;
+  volatile bool done;
   pthread_mutex_t mutex;
 };