This is a reland of 9afde91b

Original change's description:
> [builtins] Load external references from the external-reference-table
>
> Off-heap code cannot embed external references. With this CL, we load
> from the external reference table (reached through the root pointer)
> instead.
>
> In a follow-up, the table could be stored within the isolate itself,
> removing one more level of indirection.
>
> Bug: v8:6666
> Change-Id: I4c612ad3d4112ec03c3b389f5bfb9cdc3dc8a671
> Reviewed-on: https://chromium-review.googlesource.com/970468
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52073}

TBR=mstarzinger@chromium.org

Bug: v8:6666, v8:7580
Change-Id: I163cfc15605c1183b79ead77df0e37d71d60b6f7
Reviewed-on: https://chromium-review.googlesource.com/972821
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52118}