the new value to the result.

Unlike `fetch_neg`, this does not return the previous value.

`neg` takes an [`Ordering`] argument which describes the memory ordering
of this operation. All ordering modes are possible. Note that using
[`Acquire`] makes the store part of this operation [`Relaxed`], and
using [`Release`] makes the load part [`Relaxed`].

This function may generate more efficient code than `fetch_neg` on some platforms.

- x86/x86_64: `lock neg` instead of `cmpxchg` loop ({8,16,32}-bit atomics on x86, but additionally 64-bit atomics on x86_64)

# Examples

```
use portable_atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(5);
foo.neg(Ordering::Relaxed);
assert_eq!(foo.load(Ordering::Relaxed), 5_i32.wrapping_neg());
foo.neg(Ordering::Relaxed);
assert_eq!(foo.load(Ordering::Relaxed), 5);
```