Add github workflows for AoR

[TamarChristinaArm]

This adds the following:

• An automatic greeting to new contributors that asks them to check the CLA
  (note, this will not ask again, so it's weaker than a CLA bot.)
• Github actions building for native linux aarch64 gcc/clang. linux x86-64 cross
  GCC and darwin-aarch64-native clang. It looks like the darwin only supports math.
• Adds an automatic labeler based on the area that the PR is to
• Adds a CODEOWNERS file that will automatically tag the respective maintainers
  when a PR is made