VS: Provide an 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