This migrates the JSEntryStub to three dedicated builtins:

JSEntry
JSConstructEntry
JSRunMicrotasksEntry

Drive-by: Tweaks to make the code isolate-independent (e.g. using the
correct macro assembler method to load and store external references
through the kRootRegister).
Drive-by: The context slot on x64/ia32 must be set up after
kRootRegister is initialized, so we first reserve the slot and later
load its value.
Drive-by: Update all remaining comments referencing JSEntryStub.

Bug: v8:7777
Change-Id: Ie3ba17ffb3bde6f18ec1d26d778b258719b2d4ef
Reviewed-on: https://chromium-review.googlesource.com/c/1365275Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58088}