Update libffi to 3.1

Update libffi from 3.0.13 to 3.1
Disable MMAP workaround for Selinux

We disable this on Rubinius since it causes crashes. The JIT
needs to be able to create these mappings anyway, so it doesn't
create an additional depedency in FFI.
Update libffi to 3.2

Update libffi from 3.1 to 3.2
Merge pull request #3183 from gustavotemple/update-libffi-3.1

Update libffi to 3.2