gdb: use ptid_t::to_string in print_target_wait_results

gdb: use ptid_t::to_string in print_target_wait_results

The ptid_t::to_string method was introduced recently, to format a ptid_t
for debug purposes.  It formats the ptid exactly as is done in
print_target_wait_results, so make print_target_wait_results use it.

Change-Id: I0a81c8040d3e1858fb304cb28366b34d94eefe4d