gexp: 'program-file' honors the current system and cross-compilation target.

Fixes <https://bugs.gnu.org/36813>.
Reported by Jakob L. Kreuze <zerodaysfordays.sdf.org@sdf.org>.

* guix/gexp.scm (program-file-compiler): Pass #:system and #:target to
'gexp->script'.
(load-path-expression): Add #:system and #:target and honor them.
(gexp->script): Likewise.
* tests/gexp.scm ("program-file #:system"): New test.
* doc/guix.texi (G-Expressions): Adjust accordingly.