Rollup merge of #92139 - dtolnay:backtrace, r=m-ou-se

Change Backtrace::enabled atomic from SeqCst to Relaxed

This atomic is not synchronizing anything outside of its own value, so we don't need the `Acquire`/`Release` guarantee that all memory operations prior to the store are visible after the subsequent load, nor the `SeqCst` guarantee of all threads seeing all of the sequentially consistent operations in the same order.

Using `Relaxed` reduces the overhead of `Backtrace::capture()` in the case that backtraces are not enabled.

## Benchmark

```rust
#![feature(backtrace)]

use std::backtrace::Backtrace;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use std::time::Instant;

fn main() {
    let begin = Instant::now();
    let mut threads = Vec::new();
    for _ in 0..64 {
        threads.push(thread::spawn(|| {
            for _ in 0..10_000_000 {
                let _ = Backtrace::capture();
                static LOL: AtomicUsize = AtomicUsize::new(0);
                LOL.store(1, Ordering::Release);
            }
        }));
    }
    for thread in threads {
        let _ = thread.join();
    }
    println!("{:?}", begin.elapsed());
}
```

**Before:** 6.73 seconds
**After:** 5.18 seconds

0 files changed, 0 insertions, 0 deletions