gnu: gcc: Use `gcc-configure-flags-for-triplet' when cross-compiling GCC.

* gnu/packages/cross-base.scm (gcc-configure-flags-for-triplet): Move to...
* gnu/packages/gcc.scm (gcc-configure-flags-for-triplet): ... here.  New
  procedure.
  (gcc-4.7): Use it when (%current-target-system) is true.