Bug#666743: draft implementation for a Multi-Arch: foreign interface to gcc

[Matthias Klose <doko@debian.org>]

As addition to the feedback given on irc,

 - please complete the patch to cover all frontends.  while these
   might not be necessary for your bootstrap effort, I'd like to
   have all frontends be covered in an uniform way.

 - as the first step, please don't build any -for-host and -for-build
   packages yet.

 - for testing:
   - the native build should succeed without disabling any packages.
   - I don't see much value in doing a non-dfsg build, just make sure
     that the new triplet packages still suggest the non-triplet doc
     packages.
   - please test a cross build, with CROSS_ARCHS set to amd64, one
     multilib arch, and one non-multilib arch (you can inject
     packaging changes into the gcc-8-cross package without
     generating a gcc-8-source first).
   - gcc-snapshot testing is not needed. at some point we might
     want to have the triplet prefixed binaries there as well.
   - Please make the testing public, e.g.
     https://launchpad.net/~gcc-for-host/+archive/ubuntu/ppa
     Any other build service is fine as well.

Thanks, Matthias