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
