rpm: Deterministically set vendor macro entry

On an aarch64 build host, vendor is found to be "unknown", on x86 systems
it is "pc". This filters through to the PLATFORM tag in target rpms.

We saw reproducibility test failures where the PLATFORM tags in noarch
rpms were changing depending upon which host built them. Forcing the
vendor value to a consistent one makes things deterministic.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>