Free memory is marked as inaccessible, which means that it contains a
zap value and is poisoned in ASAN builds.

Before writing the unlinked sentinel, we must unpoison the memory
area in ASAN builds.

Bug: chromium:1056170
Change-Id: Ib253913cce7d62e1000d4b581bdeb13a1e19cc67
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2232541
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68206}