gexp: Add #:target parameter to 'gexp->derivation'.

gexp: Add #:target parameter to 'gexp->derivation'.

* 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.