Eliminate headers with no arch from the rpmdb on rebuild

[pmatilai]

Currently rpmdb does a very basic sanity checks on headers in the db, requiring that all headers have name, version and release. Arch is missing because it's a loophole used by gpg-pubkey pseudopackages. Once we drop support for the rpmdb keystore, gpg-pubkeys should be purged out of the rpmdb. This is easily done by just adding RPMTAG_ARCH to the list of tested tags.

[pmatilai]

As it seems we'll have to carry support for this further into the future, this would need to be conditionalized on rpmdb being the configured keystore. That might get a bit weird.