Rwlock downgrade Tracking Issue: #128203 This PR adds a `downgrade` method for `RwLock` / `RwLockWriteGuard` on all currently supported platforms. Outstanding questions: - [x] ~~Does the `futex.rs` change affect performance at all? It doesn't seem like it will but we can't be certain until we bench it...~~ - [x] ~~Should the SOLID platform implementation [be ported over](https://github.com/rust-lang/rust/pull/128219#discussion_r1693470090) to the `queue.rs` implementation to allow it to support downgrades?~~