CMake: Add -T option to choose a generator toolset
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_TOOLSET and associated variable documentation to hold the value persistently. Add a RunCMake.GeneratorToolset test to cover basic "-T" option cases. Verify that CMAKE_GENERATOR_TOOLSET is empty without -T, that -T is rejected when the generator doesn't support it, and that two -T options are always rejected.