core.atomic for ldc.

[rikki cattermole]

The only difference between dmd, ldc and gdc (in effect) is the backend.
While druntime and Phobos will be patched for other platform targets, 
over all its the same library.

The same goes for core.atomic. You should not need to know that it has 
been patched. If you can call it and it gives you the right results, you 
can assume it worked.

Oh and $ dub build --compiler=ldc2