don't set explicit arch flags

This can cause problems on custom Pythons, which should handle proper arch flags (and respect ARCHFLAGS).

Python does universal build by default, matching the build flags of Python itself, so this should be unnecessary.

If Python is *not* UB, then build will match Python, rather than attempting UB that may fail.
Merge pull request #31 from minrk/10.5-build

Don't set explicit arch flags