Test i386 chroots on x86-64 systems

This should work (and, actually, it does). It was broken on Arch, so I created this test.

Also, I modified `ARCH` handling in defaults scripts, so that we can specify i386/i686 and still get x86 chroot, amd64/x86_64 and get x86-64 chroot, and arm* for arm. This is not needed for the current distros as they all use `i386`/`amd64`/`armhf`, but Arch uses `i686`/`x86-64`/`arm7h`. Having this flexibility allows us to have a common test for all distros (otherwise we would need kludgy ifs), and we can detect invalid architectures as a bonus.

This introduces a bit of code duplication (but, arguably, it was already there in the `sed` line...), not sure if it's worth doing something about it...