gdb: make so_list::abfd a gdb_bfd_ref_ptr

Change the field from a `bfd *` to a gdb_bfd_ref_ptr to automatically
manage the reference.

Change-Id: I3ace18bea985bc194c5e67bb559eec567e258950
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>