don't set explicit arch flags

This can cause problems on non-System Pythons, which should already handle proper arch flags without assistance (and respect ARCHFLAGS).

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

If Python is *not* UB, then the build will match that of Python itself, rather than attempting a UB that may fail.

An example of this is Anaconda Python, which targets 10.5 (for some crazy reason), but cannot build ppc.