make nim bootstrap again for older versions

lib/system/inclrtl.nim

@@ -19,6 +19,11 @@
 when not defined(nimNewShared):
   {.pragma: gcsafe.}
 
+when not defined(nimImmediateDeprecated):
+  {.pragma: oldimmediate, immediate.}
+else:
+  {.pragma: oldimmediate.}
+
 when defined(createNimRtl):
   when defined(useNimRtl):
     {.error: "Cannot create and use nimrtl at the same time!".}