Compiling for mobile

[Morgandri1]

Checklist

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

Describe your request

Cross (hopefully) is functionally able to compile for iOS. my current build target is x86_64-apple-ios, but I'd like to be able to build for all major mobile platforms.

This seems easily possible with a custom image, but I don't know where to start!

Describe why this would be a good inclusion for cross

This would make cross pretty much universal for cross compilation as far as i know. I can't cross-compile openssl any other way.