Split GNU compiler information files

This moves GNU compiler flags into new-style modules

  Compiler/GNU-<lang>.cmake
  Platform/<os>-GNU-<lang>.cmake

We use language-independent helper modules

  Compiler/GNU.cmake
  Platform/<os>-GNU.cmake

to define macros consolidating the information.

Modules/CMakeGenericSystem.cmake

@@ -88,6 +88,3 @@ MARK_AS_ADVANCED(
   CMAKE_SKIP_RPATH
   CMAKE_VERBOSE_MAKEFILE
 )
-
-# always include the gcc compiler information
-INCLUDE(Platform/gcc)