Initial cross-compilation support

[Ludovic Courtès]

Hello!

Commit 827d289 of the ‘core-updates’ branch adds cross-base.scm, which
builds a cross tool chain.

So if you type ‘guix build gcc-cross-mips64el-linux-gnu’, you get a
cross-compiler (+ libc, binutils) for that platform.  The compiler is
slightly modified by this patch:

  
http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/patches/gcc-cross-environment-variables.patch?h=core-updates&id=827d28914a16ef2d10ebdad4695efdb02ace07fb

The end goal is to be able to run:

  guix build --target=mips64el-linux-gnu coreutils

and get a cross-built Coreutils.

What remains to be done is to implement ‘package-cross-derivation’ in
(guix packages), and to adjust build-system/gnu to support cross builds.

If someone volunteers, that’s always appreciated ;-), otherwise I’ll
check what I can do later on.

Ludo’.