The external pointer table is now managed by the GC, which marks entries
that are alive during major GC, then sweeps the table afterwards to free
all dead entries and build a free list from them. For now, only major GCs
are supported, Scavenger GCs do not interact with the external pointer table.

In more detail, garbage collection of the external pointer table works
as follows:

1. The external pointer table now reserves a large region of virtual
   address space for its backing buffer and is then never reallocated,
   only grown in place until the maximum size is reached.
2. When the GC's marking visitor marks a HeapObject with an external
   pointer as alive, it also marks the corresponding external pointer
   table entry as alive. This can happen on a background thread.
3. For that, it uses the MSB of each entry in the table to indicate
   whether the entry has been marked or not. This works because the MSB
   is always cleared during the AND-based type check performed when
   accessing an external pointer.
4. After marking, the external pointer table is swept while the mutator
   is stopped. This builds an inline, singly-linked freelist of all
   newly-dead and previously-free entries.
5. When allocating an entry from the table, the first entry on the
   freelist is used. If the freelist is empty, the table grows,
   populating the freelist with the new entries.
6. Every newly-allocated entry is marked as alive, and every store to an
   existing entry also automatically marks that entry as alive (by also
   setting the MSB). This simplifies the design of the table GC with
   regards to concurrency (See ExternalPointerTable::Mark).

Bug: v8:10391
Change-Id: I8877fdf5576af3761bde65298951bb09e601bd14
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3359625Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78708}