guix system: Use standard cross and native build options.

This change prevents guix system from erroring out with an ugly backtrace
when it's passed an invalid value to the “--system” or “--target”
option. It also adds the “--list-systems” and “--list-targets” options.

The manual section about guix system doesn't mention the “--target” option,
so add it there.

* guix/scripts/system (show-help): Call show-cross-build-options-help and
show-native-build-options-help.
(%options): Remove own implementation of “system” and “target” options and
use the ones in %standard-cross-build-options and
%standard-native-build-options.
* doc/guix.texi (Invoking guix system): Document “--target” option.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 3c575dad11..957b9a668e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -38021,6 +38021,11 @@ Installation Image}).
 Attempt to build for @var{system} instead of the host system type.
 This works as per @command{guix build} (@pxref{Invoking guix build}).
 
+@item --target=@var{triplet}
+Cross-build for @var{triplet}, which must be a valid GNU triplet, such
+as @code{"aarch64-linux-gnu"} (@pxref{Specifying target triplets, GNU
+configuration triplets,, autoconf, Autoconf}).
+
 @item --derivation
 @itemx -d
 Return the derivation file name of the given operating system without