core.stdc.stdatomic

[Richard (Rikki) Andrew Cattermole]

For the most part it looks like everything is in core.atomic already.

The only thing that looks like it isn't is ``atomic_signal_fence`` and 
that has to be an intrinsic as it is specifically for a backend instruction.

But reminder: only dmd is missing the intrinsics necessary to do atomics 
correctly without memory errors. Dmd is a right off if you are doing 
anything more than just reference counting with it.