Root slots are full pointer size. Using tagged ptr loads
will lead to errors on big endian platform.

Change-Id: I477597fadee025b6fb060d67a83bee60290aeb14
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2857636
Commit-Queue: Junliang Yan <junyan@redhat.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74273}