* guix/gexp.scm (lower-inputs): Add #:system and #:target. Use
'package->cross-derivation' when TARGET is true. Honor SYSTEM.
(gexp->derivation): Add #:target argument. Pass SYSTEM and TARGET to
'lower-inputs' and 'gexp->sexp'.
(gexp->sexp): Add #:system and #:target. Pass them in recursive call
and to 'package-file'.
* tests/gexp.scm (gexp->sexp*): Add 'system' and 'target' parameters.
("gexp->derivation, cross-compilation"): New test.
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