gnu: gcc: Add a RUNPATH to libstdc++.so so that it finds libgcc_s.so.

Fixes <http://bugs.gnu.org/20358>.
Reported by 宋文武 <iyzsong@gmail.com>.

* gnu/packages/gcc.scm (gcc-4.7)[arguments]: Add 'substitute*' form for
  "libstdc++-v3/src/Makefile.in".