rts: LDV profiling fixes

rts: Do not traverse nursery for dead closures in LDV profile

It is important that heapCensus and LdvCensusForDead traverse the same areas.

heapCensus increases the not_used counter which tracks how many closures are live but haven't been used yet.

LdvCensusForDead increases the void_total counter which tracks how many dead closures there are.

The LAG is then calculated by substracting the void_total from not_used and so it is essential that not_used >= void_total. This fact is checked by quite a few assertions.

However, if a program has low maximum residency but allocates a lot in the nursery then these assertions were failing (see #16753 (closed) and #15903 (closed)) because LdvCensusForDead was observing dead closures from the nursery which totalled more than the not_used. The same closures were not counted by heapCensus.

Therefore, it seems that the correct fix is to make LdvCensusForDead agree with heapCensus and not traverse the nursery for dead closures.

rts: Correct assertion in LDV_recordDead

It is possible that void_total is exactly equal to not_used and the other assertions for this check for <= rather than <.

Correct handling of LARGE ARR_WORDS in LDV profiler

This implements the correct fix for #11627 (closed) by skipping over the slop (which is zeroed) rather than adding special case logic for LARGE ARR_WORDS which runs the risk of not performing a correct census by ignoring any subsequent blocks.

This approach implements similar logic to that in Sanity.c