Previously, when accessing SandboxedPointer fields with the sandbox
disabled, we would always do a ReadUnalignedValue/WriteUnalignedValue.
However, that is only necessary when pointer compression is enabled.
Otherwise, the field will be properly aligned.

This CL also factors out the logic to determine when to use an unaligned
or aligned read/write for a field into two new helper functions.

Bug: chromium:1292669
Change-Id: I2c1af187c5b2699101c3fee9cc551be788d3a845
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3429200Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78887}