Loads target but with relaxed load. Concurrent marking needs to load
field with relaxed load, since the main thread could change this field when unregistering.

Change-Id: I809b1d4db1dd58c92bdb998601c2f709073104af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2192661Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67707}