core.atomic for ldc.

[rm]

> It look like atomicFetchAdd a atomicFetchSub was added to ldc 8 days ago 
> and my distro has old version. I replace core.internal.atomic with this 
> https://raw.githubusercontent.com/ldc-developers/druntime/ldc/src/core/internal/atomic.d 
> and now everything works.


It's not even in the latest stable version. Should be available in next 
release.
Also, please note that I haven't had the time to check if anything else 
was missing. If you do spot something else that is missing, please let 
me know.