From recent discussions on https://crrev.com/c/2057355 it became clear
that the FixedRawData serializer bytecodes need to be refactored. This
CL

- updates documentation to correctly specify that data length is given
in kTaggedSize unit, and
- changes the way smi roots are serialized s.t. deserialization no
longer needs hacks to be correct on big-endian architectures and when
(theoretically) deserializing sequences of smi roots. We do this by
serializing the full object slot for smi roots instead of just the
smi contents.

Bug: v8:10374
Change-Id: I190172d8030888e1950413e436e43b3d6e3b8fd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2130276
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#66966}