checker: obey [ref_only] tag, allow embedding in other ref struct (#8707)

vlib/sync/sync_default.c.v

@@ -28,10 +28,12 @@ fn C.sem_timedwait(voidptr, voidptr) int
 fn C.sem_destroy(voidptr) int
 
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
+[ref_only]
 pub struct Mutex {
 	mutex C.pthread_mutex_t
 }
 
+[ref_only]
 pub struct RwMutex {
 	mutex C.pthread_rwlock_t
 }
@@ -40,6 +42,7 @@ struct RwMutexAttr {
 	attr C.pthread_rwlockattr_t
 }
 
+[ref_only]
 struct Semaphore {
 	sem C.sem_t
 }