Add support for cross-building

Here is some minimal support to enable to build debian-installer on another arch than the host (ppc64el on amd64 here). I have been asked this enablement to do CI on the installer and its components without having for instance a ppc64el machine and still be able to generate the debian-installer for ppc64el especially on salsa. Which works now, but other combinations may need specific changes or not be possible at all.

For ppc64el generation on amd64, one now just need a : sbuild --on amd64 --host ppc64el --profiles=cross,nocheck

The mini.iso just worked fine. It also worked for powerpc on amd64 but I didn't test the result.