Optimize comparison in _parse_pkg_meta

This uses a more efficient method of checking if the package arch for
debian packages is one of two different arches. Inspired by a
conversation here:

https://github.com/saltstack/salt/pull/6587#issuecomment-22365073
Merge pull request #6604 from terminalmage/deb-arch-comparison-optimization

Optimize comparison in _parse_pkg_meta