Hello,

This is a follow-up of https://issues.guix.gnu.org/55220 that is still
in discussion.

If we were to create a top 10 of the most asked questions on #guix,
"What is --target, what's the difference with --system?", and "Why
--system=aarch64-linux isn't working on my machine?" would be in good
position I guess.

This patch builds upon the proposed introduction of
--list-systems/--list-targets options as well as --system/--target
argument checking to document how to build for foreign architectures.

I hope that this will shed some light on the matter.

Thanks,

Mathieu