ARM notification group

Github Label: O-ARM
Ping command: @rustbot ping arm

This list will be used to ask for help both in diagnosing and testing ARM-related issues as well as suggestions on how to resolve interesting questions regarding our ARM support.

The group also has an associated Zulip stream (#t-compiler/arm) where people can go to pose questions and discuss ARM-specific topics.

So, if you are interested in participating, please sign up for the ARM group! To do so, open a PR against the rust-lang/team repository. Just follow this example, but change the username to your own!