Go closures for x86_64

This is the patch set posted at
  https://sourceware.org/ml/libffi-discuss/2014/msg00123.html
which updates for x86_64-linux, x32-linux, and x86_64-cygwin.

Kai has done a review of the win64 bits with me on IRC.
I asked him to post it to the list as well, but...