The ctest_configure command already reads the CTEST_CMAKE_GENERATOR
variable to get the value for the cmake -G option.  Read new variable
CTEST_CMAKE_GENERATOR_PLATFORM to pass on as CMAKE_GENERATOR_PLATFORM.

The "ctest --build-and-test" mode already has "--build-generator" to
specify the -G option to CMake.  Add a "--build-generator-platform" option
to specify a value to pass on as CMAKE_GENERATOR_PLATFORM.