* guix/build-system/perl.scm: Add info on cross-compilation.
(lower)[private-keywords]: Remove #:target when cross-compiling.
(lower)[target]: Set.
(host-inputs)[perl]: New entry.
(host-inputs)[(standard-packages)]: Move to ...
(build-inputs)[(standard-packages)]: ... here when cross-compiling.
(build-inputs)[standard-cross-packages]: Add when cross-compiling.
(target-inputs): New entry when cross-compiling.
(build): Use perl-cross-build when cross-compiling.
(perl-cross-build): New procedure.
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>