Rollup merge of #134389 - rust-wasi-web:condvar-no-threads, r=m-ou-se

Condvar: implement wait_timeout for targets without threads

This always falls back to sleeping since there is no way to notify a condvar on a target without threads.

Even on a target that has no threads the following code is a legitimate use case:

```rust
use std::sync::{Condvar, Mutex};
use std::time::Duration;

fn main() {
    let cv = Condvar::new();
    let mutex = Mutex::new(());
    let mut guard = mutex.lock().unwrap();

    cv.notify_one();

    let res;
    (guard, res) = cv.wait_timeout(guard, Duration::from_secs(3)).unwrap();
    assert!(res.timed_out());
}
```

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