[scudo][standalone] Simplify populateFreelist

`populateFreelist` was more complicated that it needed to be. We used
to call to `populateBatches` that would do some internal shuffling and
add pointers one by one to the batches, but ultimately this was not
needed. We can get rid of `populateBatches`, and do processing in
bulk. This doesn't necessarily make things faster as this is not on the
hot path, but it makes the function cleaner.

Additionally clean up a couple of items, like `UNLIKELY`s and setting
`Exhausted` to `false` which can't happen.

Differential Revision: https://reviews.llvm.org/D90700