gem5: update to d4b3e064adeeace3c3e7d106801f95c14637c12f

Mostly for --param to control debug properly.

Less README workarounds, more ./run automation, oh yeah.

run

@@ -183,6 +183,9 @@ def main(args, extra_args=None):
         elif args.gem5_script == 'se':
             assert(args.exec_image is not None)
             cmd += [common.gem5_se_file, '-c', args.exec_image]
+        if args.debug_guest:
+            # https://stackoverflow.com/questions/49296092/how-to-make-gem5-wait-for-gdb-to-connect-to-reliably-break-at-start-kernel-of-th
+            cmd += ['--param', 'system.cpu[0].wait_for_remote_gdb = True']
     else:
         if not os.path.exists(common.image):
             raise_image_not_found()