gdbsupport: use "reference" and "pointer" type aliases in intrusive_list

It seems to me like the code should used the defined type aliases, for
consistency.

Change-Id: Ib52493ff18ad29464405275bc10a0c6704ed39e9
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>