Slop-zeroing thunks is problematic with nonmoving collector

To aid in sanity checking the debug RTS when run with -N1 currently zeros the free variables of thunks when they are updated. However, this is problematic for the nonmoving collector, which may race to read free variables while a thunk is being updated. I believe the fix here is to ensure that slop-zeroing is disabled while the nonmoving collector is in use, although sadly this further restricts the sanity checking that we can safely perform.