CI: Add Arm support, reorganise templates

Quite a big rework and lots of movement in the CI templates, but the end result is that we get AArch64 and ARMv7 support, and other configurations are a lot easier to add as well. There's still some shuffling to do - and patching virtme so we can pass the DTB in order to be able to boot our ARMv7 VM when running on AArch64 - but there's enough change in this series that I think it's worth posting now and dealing with the rest later.

@emersion This was my detour when looking at FreeBSD; I wanted to work on Weston first since it was more complex & parameterised, then work backwards from there to make the Wayland/FreeBSD support more similar to where this ends up going. As part of that, I wrote a ton of comments through the file, so it's hopefully much more clear what's going on.