When we exit the kernel debugger having entered because of a breakpoint

instruction we need to jump over the instruction. Without this we will
execute the same instruction again and enter into the debugger again.

PR:		223917
Reported by:	emaste
MFC after:	1 week
Sponsored by:	DARPA, AFRL

sys/arm64/arm64/trap.c

@@ -323,7 +323,10 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 			break;
 		}
 #endif
-		/* FALLTHROUGH */
+		kdb_trap(exception, 0,
+		    (td->td_frame != NULL) ? td->td_frame : frame);
+		frame->tf_elr -= 4;
+		break;
 	case EXCP_WATCHPT_EL1:
 	case EXCP_SOFTSTP_EL1:
 #ifdef KDB