gatomic: use GCC C11-style atomics, if available

GCC does not yet support ISO C11 atomic operations, but it has
compatible versions available as an extension.  Use these for load and
store if they are available in order to avoid emitting a hard fence
instruction (since in many cases, we do not need it -- on x86, for
example).

For now we use the fully seqentially-consistent memory model, since
these APIs are documented rather explicitly: "This call acts as a full
compiler and hardware memory barrier".

In the future we can consider introducing new APIs for the more relaxed
memory models, if they are available (or fall back to stricter ones
otherwise).

https://bugzilla.gnome.org/show_bug.cgi?id=730807