Define the 'cmake -A' option to set CMAKE_GENERATOR_PLATFORM
without having to spell out the whole variable name. We choose
the name '-A' for "platform" because '-P' is already taken, and
in the common use case the "platform" is actually an architecture
(e.g. x64).
Teach the RunCMake test infrastructure to use -A to pass the generator
platform. Extend the RunCMake.GeneratorPlatform test with a case to
verify that the -A option cannot be repeated.
Brad King