build: Use the canonical list of supported systems in 'assert-*'.

* build-aux/check-available-binaries.scm (%supported-systems): Remove.
* build-aux/check-final-inputs-self-contained.scm (%supported-systems):
  Remove.

build-aux/check-available-binaries.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -28,9 +28,6 @@
              (srfi srfi-1)
              (srfi srfi-26))
 
-(define %supported-systems
-  '("x86_64-linux" "i686-linux"))
-
 (let* ((store  (open-connection))
        (native (append-map (lambda (system)
                              (map (cut package-derivation store <> system)

build-aux/check-final-inputs-self-contained.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -29,9 +29,6 @@
              (srfi srfi-1)
              (srfi srfi-26))
 
-(define %supported-systems
-  '("x86_64-linux" "i686-linux"))
-
 (define (final-inputs store system)
   "Return the list of outputs directories of the final inputs for SYSTEM."
   (append-map (match-lambda