This CL replaces the AllocateExternalPointerEntries methods with
per-field init_##fieldname methods for every external pointer field.
These now initialize the field by allocating the external pointer table
entry for it and also set the initial value. This saves one memory write
in the typical case (what used to be AllocateExternalPointerEntries
followed by a set_##fieldname) and removes some code that can be
factored out into the (DECL_)EXTERNAL_POINTER_ACCESSORS macro.

Bug: v8:10391
Change-Id: Iac937316322910de0a8bb99b33592eca8c57a4a6
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/+/3810344Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82482}