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.

Change-Id: If61fd5cfa153f98757beeb444a530f6e2803fdb6
Bug: chromium:1023139
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906376
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64991}