Support amd64 as a no-op

[lalten]

It would be great if amd64 (or maybe just native?) was supported as a valid arch. It would simplify workflows by removing special-casing like
https://github.com/AppImage/type2-runtime/blob/c1ea750/.github/workflows/build.yaml#L52-L53

[martin-g]

Already worked on at #58

[lalten]

That PR seems stalled (which is why I opened #94) and has conflicts with master.

[sparr]

Dupe of #31?