Refuse to create transaction elements from headers with no RPMTAG_ARCH

[pmatilai]

gpg-pubkey pseudo-packages are a special case that (intentionally) lack the ARCH tag. This allegedly differentiates them from real packages and kinda implies they can only appear through rpmtsImportHeader(), but actually the special casing in rpmte.c lets anybody create a package with RPMTAG_ARCH stripped and install it.