Loading external references from off-heap builtins will be
root-pointer-relative. At least initially, these loads will happen in
CSA and thus need access to the root pointer value.

Bug: v8:6666
Change-Id: Iae4c89061df442f5afd03f93e5ba35c4e125b850
Reviewed-on: https://chromium-review.googlesource.com/970264Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52069}