Before this CL a byte was used per bucket to store whether the bucket
is possibly empty or not. This CL changes this such that each bucket
only needs a single bit.

PossiblyEmptyBuckets is now a word in the page header. If more bits
are needed than fit into a single word, an external bitmap is
allocated using AlignedAlloc. Storing this on the page header, allows
to remove initial_buckets from the SlotSet. The SlotSet allocation is
then again a power-of-2 in release mode.

Reland of https://crrev.com/c/1906376: Incorrect DCHECK was removed.
WordsForBuckets was simplified and a test was added for it.

Bug: chromium:1023139
Change-Id: I9a08e03a9c10e5781a146b9a28dab38824aad91f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1954391Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65385}