Add option to disable XNNPACK executor runner build (#242)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/242

As titled. This turns it off by default for CMake so that we can build faster.

Did a search didn't see this being used by anyone else so should be fine.

Reviewed By: digantdesai, huydhn, dbort

Differential Revision: D49075107

fbshipit-source-id: 5cf8ad4a6f31f793bce969ca457b34111f8b28f8