add implementation for no_threads.rs and teeos.rs

Author: connortsui20

library/std/src/sys/sync/rwlock/no_threads.rs

@@ -65,6 +65,6 @@ impl RwLock {
 
     #[inline]
     pub unsafe fn downgrade(&self) {
-        todo!()
+        assert_eq!(self.mode.replace(1), -1);
     }
 }

library/std/src/sys/sync/rwlock/teeos.rs

@@ -43,7 +43,5 @@ impl RwLock {
     }
 
     #[inline]
-    pub unsafe fn downgrade(&self) {
-        todo!()
-    }
+    pub unsafe fn downgrade(&self) {}
 }