aarch64 runners are now available in GitLab

Hi everyone,

I have repurposed aarch64 hardware that has been previously used by 
Flathub as GitLab runners on gitlab.gnome.org. These are not the fastest 
servers in the world but are more than enough to test if software builds 
properly on non-x86_64 architecture(s).

The runners consume jobs tagged aarch64 and flatpak-aarch64; as for the 
latter, there's no prebaked image with flatpak-builder and GNOME Nightly 
SDK at the time.

The hardware is donated by Codethink. Thank you!

Bart