rms.

- x86/x86_64: `lock not` instead of `cmpxchg` loop ({8,16,32}-bit atomics on x86, but additionally 64-bit atomics on x86_64)
- MSP430: `inv` instead of disabling interrupts ({8,16}-bit atomics)

# Examples

```
use portable_atomic::{AtomicI16, Ordering};

let foo = AtomicI16::new(0);
foo.not(Ordering::Relaxed);
assert_eq!(foo.load(Ordering::Relaxed), !0);
```