Auto merge of #128219 - connortsui20:rwlock-downgrade, r=tgross35

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](rust-lang/rust#128219 (comment)) to the `queue.rs` implementation to allow it to support downgrades?~~

Author: bors