Draft: d/control: Don't build gdm3 binary package on armel

Simon McVittierequested to merge
wip/not-for-armel into debian/latest

gdm3 requires GNOME Shell, which is unlikely to be practically useful on the older ARM machines that can't run armhf or arm64. GNOME Shell also requires gjs, which is likely to be removed from armel.

Helps: #1080521

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