[bug#36477] Add Guix System cross-compilation support

[Mathieu Othacehe]

Hey Guix,

I'm currently working on adding --target support to guix system
command. The main obstacle is that cross-compilation is broken on many
packages because of small errors in package definitions (inputs vs
native-inputs).

I created a branch on savannah called "wip-cross-system" to centralize
this work. Feel free to have a look / review some of the patches there!

Thanks,

Mathieu