CI: Set up CI for ARM
xref #33971

This used to run in Travis, but seems like Travis now requires credits, and we can't get them reliably (see #38943).

[This page](https://developer.arm.com/solutions/infrastructure/developer-resources/ci-cd) contains CI systems that support ARM. Most of them require to set up our own runner, which I guess we don't want to do or maintain. So, the main options seem to be:
- drone.io
- CircleCI

CC: @bashtage 