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

vlib/sync/sync_macos.c.v

@@ -31,10 +31,12 @@ fn C.pthread_cond_timedwait(voidptr, voidptr, voidptr) int
 fn C.pthread_cond_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
 }
@@ -49,6 +51,7 @@ struct CondAttr {
 
 /* MacOSX has no unnamed semaphores and no `timed_wait()` at all
    so we emulate the behaviour with other devices */
+[ref_only]
 struct Semaphore {
 	mtx C.pthread_mutex_t
 	cond C.pthread_cond_t