External references are process-specific and thus need to be accessed
through an indirection (or reloc'd by the linker). This CL moves all
used external references to the builtins constants table and rewrites
accesses to load from there.

In the future, this could be made more efficient by removing levels of
indirection or using the native linker.

Bug: v8:6666
Change-Id: I63491670549654edeb59c60bb833acfdc5a48495
Reviewed-on: https://chromium-review.googlesource.com/939783Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51646}