Add ARM64 support to VS project generation
Add ARM64 support to VS project generation

Windows builds can use msbuild.exe to build libvpx through a set of
generated Visual Studio project files. This commit adds awareness of
ARM64 Windows to this process by adding ARM64 configurations and
setting msbuild properties to consume the right SDK version.

Change-Id: I1bbc01cbe7be3d53c4e1af6cd96c6e4170aa4915

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx