We parse `CMAKE_GENERATOR_TOOLSET` values of the forms:
* `toolset`
* `toolset,host=x64`
* `host=x64`
Generalize the parsing to support the forms:
* `toolset`
* `toolset[,key=value]*`
* `key=value[,key=value]*`
Disallow duplicate keys. Require all but the first field to
be of `key=value` form.
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