libgomp: Save OpenMP device number when initializing the interop object

The interop object (opaque object to the user, used internally in libgomp)
already had a 'device_num' member, but it was missed to actually set it.

libgomp/ChangeLog:

	* target.c (gomp_interop_internal): Set the 'device_num' member
	when initializing an interop object.

libgomp/target.c

@@ -5324,6 +5324,7 @@ gomp_interop_internal (void *data)
 	    }
 	  *obj =
 	    (struct interop_obj_t *) calloc (1, sizeof (struct interop_obj_t));
+	  (*obj)->device_num = devicep->target_id;
 	  devicep->interop_func (*obj, devicep->target_id,
 				 gomp_interop_flag_init, targetsync,
 				 prefer_type);