It is not safe to allocate ExternalPointerTable entries while the table
is being swept. This property is currently ensured by the GC. To better
catch any potential future violation of this requirement, this CL now
changes the Sweep() method to first set the freelist head to a special
marker value, which is checked in Allocate() in debug builds and will
cause a recognizable crash in release builds.

Bug: v8:10391
Change-Id: Iab69c1e97afc23ae5b2b894b2d765b82a760cdd8
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/+/3758211Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81690}