On Fedora systems, it is invalid to mmap something with PROT_WRITE and
PROT_EXEC. libffi has been updated to support this, but ctypes has not
been updated to use this new functionality.

Attached is a patch which currently only works if system libffi is used.
Though the embedded version of libffi is new enough, it is missing the
allocation and free functions. I know how I would update the ctypes
libffi/ directory (add the alloc/free files), but not sure about the
other libffi directories (darwin, arm_wince, msvc, osx). I suppose those
would all need to be upgraded, or perhaps even made to use the standard
libffi instead of special forks of it.

Note that this also eliminates the malloc_closure code.