If the CMAKE_CROSSCOMPILING_EMULATOR variable is defined, and
CMAKE_CROSSCOMPILING is TRUE, then use CMAKE_CROSSCOMPILING_EMULATOR to run
the try_run executables. This prevents the need to populate
TryRunResults.cmake when cross compiling.
Most functionality is already implemented in Source/cmCoreTryCompile.{h,cxx}.
Document and improve argument parsing.
This functionality is already being used by a number of modules, like
CheckCSourceCompiles.cmake, but it is not documented.