[Issue 12891] add atomicFetchAdd and atomicFetchSub to core.atomic

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12891

--- Comment #8 from Jonathan Dunlap <jadit2 at gmail.com> ---
Thanks Martin for the gist sample, I've started to integrate in this branch:
https://github.com/jadbox/druntime/tree/fetchmod

Currently I'm having an issue where a static if check against
__traits(isSame,T,V1) is always resulting to false, even though it seems T and
V1 are the same in the unittests.

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