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/environment.scm

@@ -96,8 +96,6 @@ shell'."
   (display (G_ "
       --search-paths     display needed environment variable definitions"))
   (display (G_ "
-  -s, --system=SYSTEM    attempt to build for SYSTEM--e.g., \"i686-linux\""))
-  (display (G_ "
   -r, --root=FILE        make FILE a symlink to the result, and register it
                          as a garbage collector root"))
   (display (G_ "
@@ -145,6 +143,8 @@ COMMAND or an interactive shell in that environment.\n"))
   (newline)
   (show-build-options-help)
   (newline)
+  (show-native-build-options-help)
+  (newline)
   (show-transformation-options-help)
   (newline)
   (display (G_ "
@@ -226,10 +226,6 @@ use '--preserve' instead~%"))
          (option '(#\n "dry-run") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'dry-run? #t result)))
-         (option '(#\s "system") #t #f
-                 (lambda (opt name arg result)
-                   (alist-cons 'system arg
-                               (alist-delete 'system result eq?))))
          (option '(#\C "container") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'container? #t result)))
@@ -273,7 +269,8 @@ use '--preserve' instead~%"))
                    (alist-cons 'bootstrap? #t result)))
 
          (append %transformation-options
-                 %standard-build-options)))
+                 %standard-build-options
+                 %standard-native-build-options)))
 
 (define (pick-all alist key)
   "Return a list of values in ALIST associated with KEY."