emacs: Display supported systems in "Package Info".

* emacs/guix-main.scm (%package-param-alist): Add 'systems'.
* emacs/guix-ui-package.el (guix-package-info-insert-systems): New
  procedure.
  (guix-package-info-format): Add it.
  (guix-output-info-format): Likewise.
  (guix-package-info-titles): Add "Supported systems" title.