This is a reland of 8e39af62

Original change's description:
> [arm64] Use root register for addressing external references.
> 
> This optimization is already done on x64 (7500e507).
> 
> Bug: v8:7844
> Change-Id: Iccc3bb55aa79ef1d4423576c79d9ce6f829f2828
> Reviewed-on: https://chromium-review.googlesource.com/1120343
> Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54162}

Bug: v8:7844
Change-Id: I2eab2d753fd8e374bf7c912a107c93edc58ef4c7
Reviewed-on: https://chromium-review.googlesource.com/1126259Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#54257}