Update to the 20180504 snapshot of GCC 8.1.1. This is just after the
release of GCC 8.1 (the first of the GCC 8 series) and thus pretty much
the same, hence the jump in version number from 8.0.1 to 8.1.1.

Add a conflict with the soon forthcoming lang/gcc8 port that is going
to track GCC 8 releases.