Bring back arm64

This MR brings back the arm64 build which is currently outdated (3.36), adds CI and and makes sure scaling works on the L5 display

/cc @jeremiah.foster @matthias.klumpp since this is precedence

• i did not add changelog entries since they're easier done using gbp-dch, is that the PureOS way?
• is it o.k. to hook https://source.puri.sm/pureos/packages/gnome-control-center to our build-from-git machinery? (Librem5/deb-build-jobs!304 (merged))

(also /cc @mohammed.sadiq since he worked on Librem5's g-c-c a lot so he is aware).