allow old styled RTTI for arc/orc (#15331)

lib/system/hti.nim

@@ -98,6 +98,8 @@ type
     finalizer*: pointer # the finalizer for the type
     marker*: proc (p: pointer, op: int) {.nimcall, benign, tags: [], raises: [].} # marker proc for GC
     deepcopy: proc (p: pointer): pointer {.nimcall, benign, tags: [], raises: [].}
+    when defined(nimSeqsV2):
+      typeInfoV2*: pointer
     when defined(nimTypeNames):
       name: cstring
       nextType: ptr TNimType