For VS generator names that do not specify the platform name, read
CMAKE_GENERATOR_PLATFORM to get it.
Extend the RunCMake.GeneratorPlatform test with a case covering
use of the x64 platform when the test generator is a Visual Studio
generator whose name does not specify a platform.
Reject the option by default. It will be implemented on a per-generator
basis. Pass the setting into try_compile project generation. Add cache
entry CMAKE_GENERATOR_PLATFORM and associated variable documentation to
hold the value persistently.
Add a RunCMake.GeneratorPlatform test to cover basic use cases for the
option. Verify that CMAKE_GENERATOR_PLATFORM is empty by default, and
that it is rejected when the generator does not support a user setting.
Brad King