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>

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

guix/scripts/system.scm

@@ -1046,6 +1046,10 @@ Some ACTIONS support additional ARGS.\n"))
                          for 'describe' and 'list-generations', list installed
                          packages matching REGEXP"))
   (newline)
+  (show-cross-build-options-help)
+  (newline)
+  (show-native-build-options-help)
+  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "
@@ -1136,14 +1140,6 @@ Some ACTIONS support additional ARGS.\n"))
                    (let ((level (string->number* arg)))
                      (alist-cons 'verbosity level
                                  (alist-delete 'verbosity result)))))
-         (option '(#\s "system") #t #f
-                 (lambda (opt name arg result)
-                   (alist-cons 'system arg
-                               (alist-delete 'system result eq?))))
-         (option '("target") #t #f
-                 (lambda (opt name arg result)
-                   (alist-cons 'target arg
-                               (alist-delete 'target result eq?))))
          (option '(#\r "root") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'gc-root arg result)))
@@ -1153,7 +1149,9 @@ Some ACTIONS support additional ARGS.\n"))
          (option '(#\I "list-installed") #f #t
                  (lambda (opt name arg result)
                    (alist-cons 'list-installed (or arg "") result)))
-         %standard-build-options))
+         (append %standard-build-options
+                 %standard-cross-build-options
+                 %standard-native-build-options)))
 
 (define %default-options
   ;; Alist of default option values.