[compiler-rt] [compiler-rt] Add initial ARM64EC builtins support (PR #139279)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM.

If you think it's more than just "based on a patch by", you could also consider adding a `Co-authored-by:` to share the authoring credit in a more formal way.

https://github.com/llvm/llvm-project/pull/139279