- new utility return address, stg_ut_1_0_unreg_{info,ret}, which
  returns via Sp[1] (assuming R1 is in Sp[0], which it is in
  the unregisterised, no-REG_R1 case.

  The only 'consumer' of this entry point is the implementation
  of PrimOps.takeMVarzh_fast in an unregisterised build.