This removes one level of indirection from loading external
references, which can now be accesses through the root pointer with a
single load:

[kRootPointer + <offset from root list to desired offset in table>]

Bug: v8:6666
Change-Id: I927634993920828ff48621a0e17e5f3099782917
Reviewed-on: https://chromium-review.googlesource.com/971041
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52211}