VS: Add option to use x64 host tools
Visual Studio provides toolchains that are themselves built for 32-bit or 64-bit host architectures. By default it uses the 32-bit tools, but it can be told to prefer the 64-bit tools on 64-bit hosts. Extend the CMAKE_GENERATOR_TOOLSET specification to provide a way to request use of the 64-bit host tools.
Closes: #15622 (closed)