Finish win64.mak support for the 32mscoff model

Ping @rainers 
> The missing "c:" in the default is almost never useful. I suggest adding it, too.

That would break things for people with a non-C: system drive.

> It would be better to use $(ProgramFiles(x86)) instead, but I guess make fails on the nested parentheses.

I think `ProgramFiles(x86)` is not set on 32-bit systems.