Update to the 20181004 snapshot of GCC 7.3.1.
Add a partial fix for what was reported in PR 231804, 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.