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>
Replace `cmGlobalGeneratorFactory::GetGenerators` with a pair of methods
to split the list of generator names into those that have platforms in
the name and those that do not.
Add this generator *without* support for specifying the target
architecture in the generator name. cmake-gui will be taught
to provide a field for this, and command-line builds can use -A.
Also, teach this generator to select a default target architecture
based on the host architecture.
Fixes: #18689
Inspired-by: Egor Pugin <egor.pugin@gmail.com>
Brad King