Fix windows ARM build

[paleolimbot]

From Tomas:

Dear Maintainer,

unfortunately your package cannot be built on Windows/aarch64, because it attempts to use a Microsoft intrinsic _umulh, which is not available with Rtools (non-Microsoft toolchain).

The fix is trivial, as attached, feel free to use the patch.

I am aware that your upstream version includes a different patch from Jeroen Ooms, claiming that it is a toolchain bug. That is not true, and my patch conditions on _MSC_VER, which I think is more correct.

But the difference is not critical, either way, please do submit a version that has a variant of the fix to CRAN. It would be great if you could do it, soon, as this is a trivial change but is needed for Windows/aarch64.