Fix arm64 buildkernel target with "nooptions KDB"

Make kdb_trap in breakpoint exception handler conditional. If "options KDB"
is not present just panic with message that debugger is not enabled.

PR:		224653

sys/arm64/arm64/trap.c

@@ -323,8 +323,12 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 			break;
 		}
 #endif
+#ifdef KDB
 		kdb_trap(exception, 0,
 		    (td->td_frame != NULL) ? td->td_frame : frame);
+#else
+		panic("No debugger in kernel.\n");
+#endif
 		frame->tf_elr += 4;
 		break;
 	case EXCP_WATCHPT_EL1: