target/arc: add RTT commands

Since RTT is architecture agnostic, add support for using it on
the ARC architecture as well.

Change-Id: Icd0dec105177a1a224bfb1a63f0be5f03561b166
Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8720
Reviewed-by: zapb <dev@zapb.de>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/arc_cmd.c

@@ -906,5 +906,8 @@ const struct command_registration arc_monitor_command_handlers[] = {
 		.usage = "",
 		.chain = arc_core_command_handlers,
 	},
+	{
+		.chain = rtt_target_command_handlers,
+	},
 	COMMAND_REGISTRATION_DONE
 };