gnu: cross-base: Make `glibc-dynamic-linker' do the right thing.

* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Determine SYSTEM
  based on (%current-target-system) when it's true.
* gnu/packages/cross-base.scm (cross-gcc): Directly call
  (package-arguments gcc-4.7) from the body of `arguments'.

gnu/packages/bootstrap.scm

@@ -131,7 +131,10 @@ check whether everything is alright."
       (propagated-inputs (map rewritten-input
                               (package-propagated-inputs p)))))))
 
-(define* (glibc-dynamic-linker #:optional (system (%current-system)))
+(define* (glibc-dynamic-linker
+          #:optional (system (or (and=> (%current-target-system)
+                                        gnu-triplet->nix-system)
+                                 (%current-system))))
   "Return the name of Glibc's dynamic linker for SYSTEM."
   (cond ((string=? system "x86_64-linux") "/lib/ld-linux-x86-64.so.2")
         ((string=? system "i686-linux") "/lib/ld-linux.so.2")

gnu/packages/cross-base.scm

@@ -70,12 +70,6 @@
   "Return a cross-compiler for TARGET, where TARGET is a GNU triplet.  Use
 XBINUTILS as the associated cross-Binutils.  If LIBC is false, then build a
 GCC that does not target a libc; otherwise, target that libc."
-  (define args
-    ;; Get the arguments as if we were building for TARGET.  In particular, we
-    ;; want `glibc-dynamic-linker' to return the right thing.
-    (parameterize ((%current-system (gnu-triplet->nix-system target)))
-      (package-arguments gcc-4.7)))
-
   (package (inherit gcc-4.7)
     (name (string-append "gcc-cross-"
                          (if libc "" "sans-libc-")
@@ -89,7 +83,7 @@ GCC that does not target a libc; otherwise, target that libc."
                   (srfi srfi-26))
        #:patches (list (assoc-ref %build-inputs "patch/cross-env-vars"))
 
-       ,@(substitute-keyword-arguments args
+       ,@(substitute-keyword-arguments (package-arguments gcc-4.7)
            ((#:configure-flags flags)
             `(append (list ,(string-append "--target=" target)
                            ,@(if libc