[compiler-rt] Revert "[compiler-rt] [builtins] Remove unused/misnamed x86 chkstk functions" (PR #167303)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

See the two individual commits for more clarify on the two steps involved here. If accepted, I would merge this one by manually pushing both commits, instead of having to do a separate PR for just the file rename revert. (When viewing the final diff over both commits, it is more unclear what is going on, when one file is renamed and another one is added with the original name.)

https://github.com/llvm/llvm-project/pull/167303