gnu: gcc: Add $libdir to the RUNPATH of libvtv.so.

* gnu/packages/gcc.scm (gcc-4.9): Inherit from GCC-4.8.
  [source]: Add 'gcc-libvtv-runpath.patch'.
  (gcc-5.1): Inherit from GCC-4.9.
  [source]: Use patches from GCC-4.9.
* gnu/packages/patches/gcc-libvtv-runpath.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.