* target.h (struct gcc_target): Add new field to struct cxx: import_export_class.
* target-def.h (TARGET_CXX): Initialise the new field.
  (TARGET_CXX_IMPORT_EXPORT_CLASS): Provide a default value for the new field.
* doc/tm.texi: Document the new target hook.
* decl2.c (import_export_class): Invoke the import_export_class field in the
  gcc_target structure if it is not empty.

From-SVN: r83964