Auto merge of #54174 - parched:park, r=alexcrichton

Fix `thread` `park`/`unpark` synchronization

Previously the code below would not be guaranteed to exit when the
second unpark took the `return, // already unparked` path because there
was no write to synchronize with a read in `park`.

EDIT: doesn't actually require third thread
```
use std::sync::atomic::{AtomicBool, Ordering};
use std::thread::{current, spawn, park};

static FLAG: AtomicBool = AtomicBool::new(false);

fn main() {
    let thread_0 = current();
    spawn(move || {
        thread_0.unpark();
        FLAG.store(true, Ordering::Relaxed);
        thread_0.unpark();
    });

    while !FLAG.load(Ordering::Relaxed) {
        park();
    }
}
```

I have some other ideas on how to improve the performance of `park` and `unpark` using fences, avoiding any atomic RMW when the state is already `NOTIFIED`, and also how to avoid calling `notify_one` without the mutex locked. But I need to write some micro benchmarks first, so I'll submit those changes at a later date if they prove to be faster.

Fixes https://github.com/rust-lang/rust/issues/53366 I hope.

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