Add target_get_gdb_reg_list wrapper:

- replaces all calls to target->type->get_gdb_reg_list.
- add documentation in target_s to warn not to invoke callback directly.


git-svn-id: svn://svn.berlios.de/openocd/trunk@1964 b42882b7-edfa-0310-969c-e2dbd0fdcd60