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

[via Digitalmars-d-bugs]

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

--- Comment #7 from Martin Nowak <code at dawg.eu> ---
See attachment above or this gist
https://gist.github.com/MartinNowak/5111611ddc476eb49298 for a fetch_add
implementation on X86_64.

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