lock btc`, and the implementation calls them on x86/x86_64.

# Examples

```
use portable_atomic::{AtomicI8, Ordering};

let foo = AtomicI8::new(0b0000);
assert!(!foo.bit_toggle(0, Ordering::Relaxed));
assert_eq!(foo.load(Ordering::Relaxed), 0b0001);
assert!(foo.bit_toggle(0, Ordering::Relaxed));
assert_eq!(foo.load(Ordering::Relaxed), 0b0000);
```