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

[mcandre]

Checklist

• 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 rust-lang/rustup#3308

Describe why this would be a good inclusion for cross

Promotes automation, reduces mistakes.