pypy: build cffi extensions in lib_pypy

Fixes a problem reported on IRC where these modules can't be imported by
a different user than the user that installed Homebrew because building
the cffi extensions writes files to the Cellar. Pypy's ~~build process~~ packaging script
normally generates these but they ~~get cleaned~~ aren't invoked by Homebrew.
Ah yes, you are right, I forgot. Nice work on prioritising relocation :+1: