ISPC: add CMake support for VS generator
ISPC language support was added to cmake 3.19 but it doesn't allow to use VS generator on Windows.
Currently, it will produce the error:
CMake Error at C:/Program Files/CMake/share/cmake-3.21/Modules/CMakeDetermineISPCCompiler.cmake:10 (message): ISPC language not currently supported by "Visual Studio 16 2019" generator
Please provide support to use ISPC from Cmake using VS generator.