gnu: linux-libre: Do not call 'configuration-file' on non-Linux targets.

Starting from commit b55310603f, a command
like:

  guix build linux-libre -s i586-gnu -n

would crash on a wrong-type-arg error in 'kernel-config' because it was
getting #f as the 'arch' argument.  This commit fixes it.

* gnu/packages/linux.scm (make-linux-libre*)[inputs]: Check whether
'platform-linux-architecture' returns #f, as is the case for example on
i586-gnu, before calling CONFIGURATION-FILE.

gnu/packages/linux.scm

@@ -793,12 +793,14 @@ for ARCH and optionally VARIANT, or #f if there is no such configuration."
        ("mpfr" ,mpfr)
        ("mpc" ,mpc)
 
-       ,@(match (and configuration-file
-                     (configuration-file
-                      (platform-linux-architecture
+       ,@(match (let ((arch (platform-linux-architecture
                              (lookup-platform-by-target-or-system
-                        (or (%current-target-system) (%current-system))))
-                      #:variant (version-major+minor version)))
+                              (or (%current-target-system)
+                                  (%current-system))))))
+                  (and configuration-file arch
+                       (configuration-file
+                        arch
+                        #:variant (version-major+minor version))))
            (#f                                    ;no config for this platform
             '())
            ((? string? config)