Handle the arm64 unknown exception separately

Rather than falling through to the default case handle the unknown
exception with its own panic message. As ESR_EL1 is zero for this
exception stop printing it.

Sponsored by:	Arm Ltd

sys/arm64/arm64/trap.c

@@ -549,9 +549,11 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 	case EXCP_UNKNOWN:
 		if (undef_insn(1, frame))
 			break;
-		printf("Undefined instruction: %08x\n",
+		print_registers(frame);
+		print_gp_register("far", far);
+		panic("Undefined instruction: %08x",
 		    *(uint32_t *)frame->tf_elr);
-		/* FALLTHROUGH */
+		break;
 	default:
 		print_registers(frame);
 		print_gp_register("far", far);