* guix/gexp.scm (lower-object): Set target argument to 'current by default and
look for the current target system at bind time if needed,
(gexp->file): ditto,
(gexp->script): ditto,
(lower-gexp): make sure lowered extensions are not cross-compiled.
* tests/gexp.scm: Add cross-compilation test-cases for gexp->script and
gexp->file with a target passed explicitely and with a default target.
