In the doctests, only use the windows.h functions on Windows

I *think* this is the correct define to test for, but I'm not sure.

This fixes a build failure I was seeing on Ubuntu (but should affect any non-Windows build).

Also, the correct define for x86-64 is x86_64_HOST_OS (it was x64… before). This may have masked this failure on x86-64, because that branch would never have been chosen. Also fixed here.