architecture/oci-platform.go - expand to support more architecture variants

[grooverdan]

https://specs.opencontainers.org/image-spec/image-index/#platform-variants includes variants for most platforms now.

As a pre-requisite to docker-library/official-images#22107, bashbrew needs to support the addition variants.

[tianon]

As I've noted in docker-library/official-images#22107 (comment), we need support for architecture variants in more places outside DOI before we can reasonably commit to them within DOI. 😅

This is also more complicated here than simply adding more explicit architectures to our matrix -- for example, it would be very reasonable for a Go-based project to decide to compile themselves explicitly for amd64 v3+, but still be FROM debian (amd64 v1), so we really need to expand the whole build system to understand the "support vectors" for this to work correctly end-to-end.

For the specific case in #22107, this would only do anything if UBI then also updates their images to include variant. Even if they do so, the end result benefit is that users get a "404" style error messages instead of successfully downloading an image that doesn't successfully run, so I wonder whether that's actually better or whether we also need to improve the error message from tools like containerd and Docker to note which platforms it did find vs which platforms our current system is detected to support (so that the mismatch is more clear).