[llvm] Add mlir-emitc subscribers to watch the Cpp target (PR #65603)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph approved this pull request.

FYI: we're moving away from CODEOWNERS to manage subscription in favor of labels instead, but feel free to land this for now! We'll like migrate this file anyway

https://github.com/llvm/llvm-project/pull/65603