gatomic: Use new __atomic_*() intrinsics for all atomic operations
gatomic: Use new __atomic_*() intrinsics for all atomic operations

Previously we used the old `__sync_fetch_*()` intrinsics for some of the
atomic operations, such as `g_atomic_int_compare_and_exchange()`. When
available, use the new `__atomic_*()` intrinsics for those instead.

As with the rest of our use of `__atomic_*()` intrinsics, we use the
`__ATOMIC_SEQ_CST` memory model. If people want to use a less
restrictive memory model to get better performance in certain
situations, they can use the compiler intrinsics directly themselves.
`g_atomic_*()` aim to be as fast as possible while remaining general
purpose.

Tested using:
```
meson test --repeat 1000 atomic atomic-test
```

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1750

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib