[libc] workaround gcc bitfield assignment warning

Author: Yifan Zhu

libc/src/__support/threads/linux/rwlock.h

@@ -370,8 +370,9 @@ class RwLock {
 public:
   LIBC_INLINE constexpr RwLock(Role preference = Role::Reader,
                                bool is_pshared = false)
-      : is_pshared(is_pshared), preference(static_cast<unsigned>(preference)),
-        state(0), writer_tid(0), queue() {}
+      : is_pshared(is_pshared),
+        preference(static_cast<unsigned>(preference) & 1u), state(0),
+        writer_tid(0), queue() {}
 
   [[nodiscard]]
   LIBC_INLINE LockResult try_read_lock() {