add cross-compiled binaries to releases

[cyphar]

We need a make cross and we should cross-compile for releases.

[thesayyn]

hey, @cyphar would you accept a PR for this? I believe it would be easier for me if I do this with goreleaser but I am not sure what your take would be on that.

[cyphar]

For CI? That would probably be okay, though I suspect that it might be better to take the changes made to release.sh in github.com/opencontainers/runc -- I made quite a few updates which make my particular release workflow (build, sign locally using my smart card PGP key, upload) easier.

[thesayyn]

hmm, do you think it is worth creating this maintenance burden in this repository as well? I am happy to go that road but bash scripts are hard to maintain.

goreleaser supports singing via pgp (see: https://goreleaser.com/customization/sign/). we could possibly make it the default pipeline for releases.

wdyt?