[scudo][standalone] Fuchsia related fixes

While attempting to roll the latest Scudo in Fuchsia, some issues
arose. While trying to debug them, it appeared that `DCHECK`s were
also never exercised in Fuchsia. This CL fixes the following
problems:
- the size of a block in the TransferBatch class must be a multiple
  of the compact pointer scale. In some cases, it wasn't true, which
  lead to obscure crashes. Now, we round up `sizeof(TransferBatch)`.
  This only materialized in Fuchsia due to the specific parameters
  of the `DefaultConfig`;
- 2 `DCHECK` statements in Fuchsia were incorrect;
- `map()` & co. require a size multiple of a page (as enforced in
  Fuchsia `DCHECK`s), which wasn't the case for `PackedCounters`.
- In the Secondary, a parameter was marked as `UNUSED` while it is
  actually used.

Differential Revision: https://reviews.llvm.org/D100524

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