[PA] Only poison memory in the thread cache with freelist hardening.

Change-Id: I0c65550add001e4e7e9112a2a3d508135b20766f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115986


Reviewed-by: Thiabaud Engelbrecht <thiabaud@google.com>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#914746}