Bug#878356: cross-gcc-dev should support gcc-8

[Dima Kogan]

I just uploaded a new build of cross-gcc-dev. The patches apply and I
can build a set of cross-compiler packages for HOST=armhf. Something's
odd, and the packages aren't currently installable (clashing files in
/usr/share/doc in the gcc-8 and cpp-8 packages). I'm looking into that.