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
normally generates these but they get cleaned by Homebrew.
My understanding is that the primary effect of importing these is to generate pyc files, which aren't relocatable, but I'll double check.