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/graph.scm

@@ -39,7 +39,9 @@
                           options->transformation
                           %transformation-options))
   #:use-module ((guix scripts build)
-                #:select (%standard-build-options))
+                #:select (%standard-build-options
+                          %standard-native-build-options
+                          show-native-build-options-help))
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
@@ -504,10 +506,6 @@ package modules, while attempting to retain user package modules."
          (option '(#\e "expression") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'expression arg result)))
-         (option '(#\s "system") #t #f
-                 (lambda (opt name arg result)
-                   (alist-cons 'system arg
-                               (alist-delete 'system result eq?))))
          (find (lambda (option)
                 (member "load-path" (option-names option)))
               %standard-build-options)
@@ -519,7 +517,8 @@ package modules, while attempting to retain user package modules."
                  (lambda args
                    (show-version-and-exit "guix graph")))
 
-         %transformation-options))
+         (append %transformation-options
+                 %standard-native-build-options)))
 
 (define (show-help)
   ;; TRANSLATORS: Here 'dot' is the name of a program; it must not be
@@ -540,8 +539,6 @@ Emit a representation of the dependency graph of PACKAGE...\n"))
       --path             display the shortest path between the given nodes"))
   (display (G_ "
   -e, --expression=EXPR  consider the package EXPR evaluates to"))
-  (display (G_ "
-  -s, --system=SYSTEM    consider the graph for SYSTEM--e.g., \"i686-linux\""))
   (newline)
   (display (G_ "
   -L, --load-path=DIR    prepend DIR to the package module search path"))
@@ -553,6 +550,8 @@ Emit a representation of the dependency graph of PACKAGE...\n"))
   (display (G_ "
   -V, --version          display version information and exit"))
   (newline)
+  (show-native-build-options-help)
+  (newline)
   (show-bug-report-information))
 
 (define %default-options