appdata: Include compatible arches in main arch when unique
appdata: Include compatible arches in main arch when unique

For example, if a i386 build is in the repo but no x86-64 version then
also add the i386 build to the x86-64 appstream data.  However, don't
add it if that would cause a duplicate (i.e. both the x86-64 and i386
version).

Closes: #1585
Approved by: alexlarsson

flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak