Fix MSVC arm build.

There are two main issues with MSVC+ARM

1. All intrinsic functions are macros, which complicates calling other functions in arguments (e.g. use of commas, <>)
2. All vector types are actually aliases of one of _n64, _n128, so we need to use eigen_packet_wrapper and add some explicit conversions.

Fixes #2607 (closed).