- gcdIntegerIntzh_fast: another primop with mismatched return
  convention.
- tidy up PerformTake() for non-REG_R1 case.
- forkzh_fast and myThreadIdzh_fast: I mistakenly had ThreadId#s
  down as something unboxed; they're not, but unpointed. Update
  primops to use the RET_P() macro upon return instead.