MSVC SSE fixes

Here's a few fixes for issues I noticed while trying to get Mesa building on ARM using MSVC.

The most important part is the second patch, which prevents a compilation-issue due to Mesa assuming MSVC implies SSE-support. This is obviously not true any longer, as Windows runs on ARM as well these days.

The first patch is kinda unrelated, but I noticed it while poking into this, so I though I'd just throw it into the mix.

The third patch enables some more SSE code-paths on MSVC that were previously missed.

The last patch fixes a compilation issue with MSVC, where _BitScanReverse64 is attempted used on ARM, where it's not available.