This moves |root_register_value| parameter of JSEntryFunction to the
first. I.e. the type of entry function will be changed from
 Object*(Object* new_target, Object* target, Object* receiver,
         int argc, Object*** args,
         Address root_register_value)
to
 Object*(Address root_register_value,
         Object* new_target, Object* target, Object* receiver,
         int argc, Object*** args),
and moves all parameter handling except for |root_register_value| from
JSEntryVariant to JSEntryTrampolineHelper.

This is a preparation to add another JS entry point for RunMicrotasks,
whose type will be
 Object*(Address root_register_value, MicrotaskQueue*).
The new entry point requires |root_register_value| to be the first to
share the implementation of the EntryFrame setup with existing ones.

Bug: v8:8124
Change-Id: I675376a2ccd240f61cf04eea6fe9a91031e06ede
Reviewed-on: https://chromium-review.googlesource.com/c/1372857
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58683}