Give Toolchain files an easy way to access the VS 2019 default configuration
I am trying to update VCPKG to fully support Visual Studio 2019, and am running into an issue. Unlike previous versions of VS, when the 2019 generator is selected it defaults to the host platform if a CMAKE_GENERATOR_PLATFORM is not specified. We need to be able to see which platform CMake is generating for in order to figure out the right dependencies. However, no variables stating what platform is being generated seems to be set until after toolchain files run, which is making adding proper support difficult. CMAKE_HOST_SYSTEM_PROCESSOR gets close, but requires awkward mapping, and I feel like it could break in the future.