[compilation controls] Ability to easily target other CPU architectures

## Problem

There isn't an easy GUI way of targeting other architectures that flatpak can support. Optimally AppCenter has the ability to autocompile for other archs automatically when submitting an app, so that there is no friction between devices elementary is officially supported on (especially in the future)

## Proposal

There are tickboxes to set the archs compiling for when setting up an app's build settings.
I'm not 100% sure how it works but flatpak docs say that it can be specified with ` --arch=ARCH `, but it probably requires extra work if not using libs in io.elementary.Sdk (the example on the docs has a part in the manifest that adds/remove build options based on the arch).
Either way, the default arches (x86_64, ARM and Pi) could all be supported by the runtime and have whatever extra manifest stuff necessary for building as a part of the app template.

## Prior Art
Like this, but not a dropdown: