build: Put RPMs in architecture-specific subdirectories

Until now we have been putting all the RPMs we build in
RPMS/x86_64.   Now that we also build noarch RPMs, we
should have a more appropriate directory structure.

NB: After merging this change you will need to update /etc/mock/xenserver.cfg on your build machine and /etc/yum/repos.d/xapi.repo on any machines which are using this repository.