invalid cache if architecture set doesn't match

invalid cache if architecture set doesn't match

The cache heavily depends on the architecture(s) it is build for,
especially if you move from single- to multiarch. Adding a new
architecture to dpkg therefore has to be detected and must invalidate
the cache so that we don't operate on incorrect data.

The incorrect data will prevent us from doing otherwise sensible
actions (it doesn't allow bad things to happen) and the recovery is
simple and automatic in most cases, so this hides pretty well and is
also not as serious as it might sound at first.

Closes: 745036