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.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #97 from euanh/build-improvements

build: Put RPMs in architecture-specific subdirectories