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

[via Digitalmars-d-bugs]

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

--- Comment #13 from github-bugzilla at puremagic.com ---
Commit pushed to stable at https://github.com/D-Programming-Language/druntime

https://github.com/D-Programming-Language/druntime/commit/c5f10192bb88255beec572bfc2e3510267ecd766
Merge pull request #1208 from jadbox/fetchmod

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