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 (closed)