Only allow x86_64 for OS X builds

This prevents the OS X target from being picked up during iOS simulator
builds (as seen in
https://github.com/github/Archimedes/pull/34#issuecomment-57824037).
Merge pull request #1 from github/restrict-osx-valid-arches

Only allow x86_64 for OS X builds
Merge remote-tracking branch 'Quick/master'
Merge pull request #44 from github/upstream-valid-archs

Only allow x86_64 for OS X builds