FROMGIT: rust: sync: add `CondVar::notify_sync`

Wake up another thread synchronously.

This method behaves like `notify_one`, except that it hints to the
scheduler that the current thread is about to go to sleep, so it should
schedule the target thread on the same CPU.

This is used by Rust Binder as a performance optimization. When sending
a transaction to a different process, we usually know which thread will
handle it, so we can schedule that thread for execution next on this
CPU for better cache locality.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Tiago Lam <tiagolam@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240108-rb-new-condvar-methods-v4-1-88e0c871cc05@google.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
(cherry picked from commit 3e645417
 https://github.com/Rust-for-Linux/linux.git

 rust-next)
Bug: 324206405
Change-Id: I9016c1f58fe4425bb16e10bae463d9b05cd57667
Signed-off-by: Alice Ryhl <aliceryhl@google.com>