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

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

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

```
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 || {
        FLAG.store(true, Ordering::Relaxed);
        thread_0.unpark();
    });

    let thread_0 = current();
    spawn(move || {
        thread_0.unpark();
    });

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