scripts: Add --list-systems and --list-targets options.

Also factorize the --system and --target build options. Check that the passed
system and target arguments are known platforms.

* doc/guix.texi (Additional Build Options): Document the new --list-systems
and --list-targets options.
* guix/scripts/build.scm (show-cross-build-options-help,
show-emulated-build-options-help, list-systems, list-targets): New procedures.
(%standard-cross-build-options, %standard-emulated-build-options): New
variables.
(show-help): Remove --system and --target entries and use
show-cross-build-options-help and show-emulated-build-options-help procedures instead.
(%options): Remove --system and --target entries and use
%standard-cross-build-options and %standard-emulated-build-options variables instead.
* guix/scripts/archive.scm (show-help, %options): Adapt them.
* guix/scripts/environment.scm: Ditto.
* guix/scripts/graph.scm: Ditto.
* guix/scripts/pack.scm: Ditto.
* guix/scripts/pull.scm: Ditto.
* guix/scripts/size.scm: Ditto.
* guix/scripts/weather.scm: Ditto.

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

guix/scripts/pull.scm

@@ -116,12 +116,13 @@ Download and deploy the latest version of Guix.\n"))
   -p, --profile=PROFILE  use PROFILE instead of ~/.config/guix/current"))
   (display (G_ "
   -v, --verbosity=LEVEL  use the given verbosity LEVEL"))
-  (display (G_ "
-  -s, --system=SYSTEM    attempt to build for SYSTEM--e.g., \"i686-linux\""))
   (display (G_ "
       --bootstrap        use the bootstrap Guile to build the new Guix"))
   (newline)
   (show-build-options-help)
+  (newline)
+  (show-native-build-options-help)
+  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "
@@ -182,10 +183,6 @@ Download and deploy the latest version of Guix.\n"))
                  (lambda (opt name arg result)
                    (alist-cons 'profile (canonicalize-profile arg)
                                result)))
-         (option '(#\s "system") #t #f
-                 (lambda (opt name arg result)
-                   (alist-cons 'system arg
-                               (alist-delete 'system result eq?))))
          (option '(#\n "dry-run") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'dry-run? #t result)))
@@ -206,7 +203,8 @@ Download and deploy the latest version of Guix.\n"))
                  (lambda args
                    (show-version-and-exit "guix pull")))
 
-         %standard-build-options))
+         (append %standard-build-options
+                 %standard-native-build-options)))
 
 (define (warn-about-backward-updates channel start commit relation)
   "Warn about non-forward updates of CHANNEL from START to COMMIT, without