[llvm] workflows/pr-subscriber: Handle libc++ and libc++abi labels (PR #66029)

[Louis Dionne via llvm-commits]

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

LGTM. Would you prefer instead that we rename the labels to libcxx and libcxxabI?

https://github.com/llvm/llvm-project/pull/66029