Finish win64.mak support for the 32mscoff model

Ping @rainers 
> Why? I don't think it's even possible to install VS in a way that this would ever be useful

Actually I have an extracted a few files from VS2010 to get a small reference version that works building and debugging phobos (65 MB), and the folders are actually named differently ;-)
I think using  `VCBIN_SUBDIR=bin` is even a bit more obvious than specifying an empty argument.