This changes Call(ExternalReference) to use the correct Mov() overload,
which contains logic to load external references indirectly (i.e. in an
isolate-independent way).

Bug: v8:6666, v8:7985
Change-Id: Ib6f3931308c7d13fd4ef0563b7e1e83db3a760eb
Reviewed-on: https://chromium-review.googlesource.com/1152730
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54813}