Add global generator factory method to get list of known platforms

Add a `cmGlobalGeneratorFactory::GetKnownPlatforms` method to return
a list of known possible values for `CMAKE_GENERATOR_PLATFORM`.
Implement the method for each generator by referencing the list of
possible values documented in `Help/generator/*.rst` for it.

Co-Author: Julien Jomier <julien.jomier@kitware.com>