Print the instruction for the unknown exception

The arm64 unknown exception will be raised when we execute an
instruction that id invalid or disabled. To help debug these print
the instruction that failed.

Sponsored by:	The FreeBSD Foundation

sys/arm64/arm64/trap.c

@@ -492,6 +492,8 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 	case EXCP_UNKNOWN:
 		if (undef_insn(1, frame))
 			break;
+		printf("Undefined instruction: %08x\n",
+		    *(uint32_t *)frame->tf_elr);
 		/* FALLTHROUGH */
 	default:
 		print_registers(frame);