gdb: make clear_so a method of struct so_list

... just because it seems to make sense to do so.

Change-Id: Ie283c92d9b90c54e3deee96a43c6a942d8b5910b
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>