Support VARIANT on 32-bit and 64-bit architectures

* Changed definition of VARIANT struct so it matches
the 32-bit and 64-bit versions of the Windows VARIANT type
depending on GOARCH (i.e. use `int` instead of `int64` so that
the size is correct for both `386` and `amd64`)
* Added NewVariant constructor to simplify VARIANT creation
and handle special-cases when running on 32-bit related
to struct layout differences on 32-bit vs. 64-bit
Shouldn't you keep the comment and the original code? This code was originally placed to fix an odd bug between the behavior of 32-bit and 64-bit. Reverting the change without the original poster's input would be unwise.