[llvm] [Support] Add missing LLVM_ABI annotations in Atomic.h (PR #152768)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

@andrurogerz As for why this had been missed; I had the understanding that the current annotations were made with some automated tool - shouldn't that have caught these declarations? They should be available and exported in all builds, even if most users (with `LLVM_THREADING_USE_STD_CALL_ONCE=1`) don't end up using them?

https://github.com/llvm/llvm-project/pull/152768