Add arm64 os variants

[xrl]

Work towards #577 and based on documentation from the arm64 public release announcement

[xrl]

I added to the allow list of platforms and now I'm hitting some odd behavior inside the github actions environment, specifically there's a ruby but no bundle. and the gem home appears to be superuser only.

runner@fv-az1504-216:~/work/setup-ruby/setup-ruby$ gem install json -v 2.2.0
Fetching json-2.2.0.gem
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /var/lib/gems/3.0.0 directory.

and

runner@fv-az1504-216:~/work/setup-ruby/setup-ruby$ bundle install
Command 'bundle' not found, but can be installed with:
snap install ruby          # version 3.4.1, or
apt  install ruby-bundler  # version 2.3.5-2
See 'snap info ruby' for additional versions.

The GHA runtime is not controlled by a dockerfile, so I tracked down the ruby install to this script: https://github.com/actions/runner-images/blob/main/images/ubuntu/scripts/build/install-ruby.sh

[eregon]

Right, that's just the system ruby, so probably there are no Rubies in the hostedtoolcache currently (to be double-checked), since https://github.com/actions/runner-images/blob/main/images/ubuntu/scripts/build/install-ruby.sh uses builds from ruby-builder, and we haven't any for linux-arm64 yet.

I think I will just make my own PR when the other parts are ready, the changes in setup-ruby are not the hard part (building tons of rubies on this new images is), but also more is needed than what's in this PR, and it needs to be coordinated with building rubies, e.g. some older versions likely don't support linux-aarch64.
IOW, it's more work for me to integrate this PR than making my own, so let me close it.