Add support for atomicFetchAdd and atomicFetchSub

[rymrg]

Add support for atomicFetchAdd and atomicFetchSub.

https://dlang.org/phobos/core_atomic.html#.atomicFetchAdd
https://dlang.org/phobos/core_atomic.html#.atomicFetchSub

[kinke]

Oh I didn't realize there are public wrappers in core.atomic now (apparently untested...); IIRC, they started out as helper functions in core.internal.atomic for core.atomic.atomicOp(), where LDC doesn't need them, so I left them out back then.

[rymrg]

You're right. If I don't use a self built version they don't work. How can I help in adding this for next version? Atomic variables cannot be implemented without this.

/usr/include/dlang/ldc/core/internal/atomic.d(32,28): Error: llvm_atomic_store cannot be interpreted at compile time, because it has no available source code

[kinke]

How can I help in adding this for next version?

This PR should suffice; please fix the indentation, add an empty line inbetween both new functions, and move them after atomicStore as for the DMD block.

Atomic variables cannot be implemented without this.

atomicOp!"+=" and atomicOp!"-=" don't work? They are tested.

While looking at the upstream code, I've noticed a copy-paste error - atomicFetchSub for pointers using atomicFetchAdd: https://github.com/dlang/druntime/blob/13e358e253e6a420dcfc83244c9479f2db6e8178/src/core/atomic.d#L199
It'd be great if you could fix that upstream too.

[rymrg]

Done. I'm not sure how I got the error message from earlier as I cannot reproduce it right now (it was with atomicStore which is required for atomic assignment).
For reference, I also fixed the upstream issue [dlang/pull/3342].

[kinke]

Thx!