Support 64-bit Windows

[3noch]

I can't use stack until it supports two things:

1. 64-bit GHC on Windows
2. Compiling the same code for both 32-bit and 64-bit (possibly with some flag to specify which)

I couldn't find a way to accomplish (2) but it might already be in place. (1) is disabled for some reason ATM. I think @ndmitchell has fears about 64-bit GHC's stability on Windows? I personally have never had issues (other than some libraries not fully supporting 64-bit, but that's not GHC's fault).

[ndmitchell]

I have enough concerns about 64bit windows that if you only support one binary download, it should be 32bit. But you should support both ideally :) Partly its just newer and historically had some issues (the first version was dire) - now I suspect it's just less tested.

[3noch]

@ndmitchell I'm curious how long it's been since that first version. Did it produce incorrect code?

[ndmitchell]

A long time! I think it segfaulted with NAG dlls or something, pretty rare and a long time ago. Now it's just less tested.

[snoyberg]

Alright, I think I've got this working. Try running with the --arch x86_64 flag, or adding arch: x86_64 to your stack.yaml file. I'll test this out on my Windows machine today and see how it goes.

[3noch]

I've successfully built a project with both 32- and 64-bit GHC! I still need to test installing GHC.