langhooks.h, [...]: introduce new langhook...

       * langhooks.h, langhooks-def.h: introduce new langhook,
       final_write_globals, with write_global_declarations as default.
       * toplev.c: Move invocation of wrapup_global_declarations from
       compile_file to new function, write_global_declarations.  Change
       compile_file to use final_write_globals hook.  Change
       wrapup_global_declarations so writing to DECL_DEFER_OUTPUT is
       conditional.

       * cp/cp-lang.c: Change lang hooks so that final_write_globals does
       nothing for C++.
       * cp/decl.c (wrapup_globals_for_namespace): Remove special
       handling of global namespace.

From-SVN: r63051