gdb: rename struct so_list to shobj

Now that so_list lists are implemented using intrusive_list, it doesn't
really make sense for the element type to be named "_list".  Rename to
just `struct shobj` (`struct so` was deemed to be not greppable enough).

Change-Id: I1063061901298bb40fee73bf0cce44cd12154c0e
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>