build: Use the correct architecture when building packages

Previously we tagged all packages with the build machine architecture,
which has always been x86_64.  Now we have some noarch packages, which
specify 'BuildArch: noarch' in their SPEC files.   With this change,
we use the architecture which RPM gives us instead of assuming the
architecture of the build machine.

Signed-off-by: Euan Harris <euan.harris@citrix.com>