This struct represents the freelist of an ExternalPointerTable and
contains both the size and the head of the freelist. It is encoded and
stored as a single Atomic64 field (freelist_) inside the
ExternalPointerTable class. This ensures that the freelist head and size
are always synchronized.

Previously, the freelist size was encoded in freelist entries in the top
bits. This only works as long as the maximum table size is relatively
small however, as it requires both the freelist size and the index of
the next entry on the list to fit into 24 bits. To allow for bigger
maximum table sizes in the future, this CL moves the freelist size
directly into the table as part of the freelist_ field.

Bug: v8:10391
Change-Id: Id09c9b28d09d79b704ac47e6566029cfb209ecd1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3891256
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83193}