RFC: WIP: support build for (native) win64

Opening this as a place to discuss the absurdity that is windows 64. Please do not comment about how useful this will be, etc. I already know that, but there is significant amount of work that needs to be done to get this into a working state.

DO NOT MERGE. IT WILL BREAK EVERYTHING EVERYWHERE

dependancies:
 - on linux, see windows cross-compile thread. nothing new here. it works, so far as this patch goes, then you can copy the result to windows and continue from there for development/testing

 - on windows:
you will need 64-bit msys and 64-bit mingw *cross-compiler* (since msys isn't expected to ever be 64-bit compiled). i am not responsible for deviations from these directions. just don't do it.

delete existing mingw from your system (save C:/MinGW/msys/1.0/home, if you wish) to avoid conflicts

download/extract mingw to C:/MinGW (or similar location) from http://sourceforge.net/projects/mingwbuilds/files/external-binary-packages/

download/extract msys to C:/MinGW/msys (or similar location) from http://sourceforge.net/projects/mingwbuilds/files/host-windows/releases/4.7.2/64-bit/threads-win32/sjlj/