VS: Provide the default platform name to project code

The value of CMAKE_VS_PLATFORM_NAME is computed by Visual Studio generators based on CMAKE_GENERATOR_PLATFORM or some default. Prior to the VS 2019 generator, the default was always Win32. However, for the Visual Studio 16 2019 generator, the default is based on the host platform.

Store the default in a new CMAKE_VS_PLATFORM_NAME_DEFAULT variable for use by project code. This is particularly useful in toolchain files because they are allowed to set CMAKE_GENERATOR_PLATFORM and so CMAKE_VS_PLATFORM_NAME is not yet known. Of course the toolchain file author knows whether it will set CMAKE_GENERATOR_PLATFORM, and if not then CMAKE_VS_PLATFORM_NAME_DEFAULT provides the platform name that will be used.

Fixes: #19177 (closed)