request: CLI flag to list tier 1 & 2 target triples

[mcandre]

Checklist

• [X] I've looked through the issues and pull requests for similar request
• [ ] This feature could be solved with a custom docker image (optional)

Describe your request

I would love an automated way to enumerate the tier 1 and 2 target triples, so that I can loop over the mostly-working triples and avoid the more fringe triples.

Relates to https://github.com/rust-lang/rustup/issues/3308

Describe why this would be a good inclusion for cross

Promotes automation, reduces mistakes.