buildsys: Add ability for variants to support specific architectures

[zmrow]

What I'd like:
Authors of variants should be able to choose whether or not they will support an architecture. The variant should only build for architectures specified. Ideally, this would be something that could be specified in the variant's Cargo.toml, similar to the image-format.

supported_arches = ["x86_64", "aarch64"]

Any alternatives you've considered:
We could add additional logic into Makefile.toml to outlaw certain combinations of arch/variant, but this would get out of control quickly and we certainly don't need that many more lines of bash. 😄