Update to the 20181003 snapshot of GCC 6.4.1.

Add a partial fix for what was reported in PR 231804, specifically the
part that's actually relevant upstream and that I made there, cf.
https://gcc.gnu.org/ml/gcc-patches/2018-10/msg00076.html as a new local
patch files/patch-libgfortran until I have backported this to the upstream
release branch and hence we get it via the weekly snapshots.

PR: 231804