bug#28243: Guix cross-compilation doesn't work

[Manolis Ragkousis]

Hello everyone,

In current master (72e2815d18ad688b0a16ce3b3efba1172423cec4)
cross-compilation doesn't work.
Running `guix build --target=i586-pc-gnu -c8 bootstrap-tarballs' or
running `guix build --target=i686-linux-gnu -c8 bootstrap-tarballs'
fail with

phase `set-paths' succeeded after 0.0 seconds
starting phase `install-locale'
warning: failed to install 'en_US.utf8' locale: Invalid argument
phase `install-locale' succeeded after 0.0 seconds
starting phase `unpack'
In execvp of tar: No such file or directory
phase `unpack' failed after 0.0 seconds

http://paste.lisp.org/display/354383

The problem appeared both in master and core-updates before today's
merge. I think the last time I checked if cross-compilation works, must
have been in early July. I am looking for the last working commit.

Manolis