This reverts commit 8e39af62.

Reason for revert: prevent v8 roll to chromium.

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}

TBR=sigurds@chromium.org,georgia.kouveli@arm.com

Change-Id: I08801917164e42c99a14a5e767d5c034f6979e87
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7844
Reviewed-on: https://chromium-review.googlesource.com/1124996Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54188}