rts: gc stats: account properly for copied bytes in sequential collections
Fix #21745.
There is little to no test coverage
commit message:
rts: gc stats: account properly for copied bytes in sequential collections
We were not updating the [copied,any_work,scav_find_work, max_n_todo_overflow]
counters during sequential collections. As well, we were double counting for
parallel collections.
To fix this we add an `else` clause to the `if (is_par_gc())`.
The par_* counters do not need to be updated in the sequential case
because they must be 0.