This reverts commit 6031412e.

Reason for revert: Tanks quite a few benchmarks (crbug.com/827854).

Original change's description:
> [x64] Reuse scratch register for ExternalReference
> 
> This reduces CEntryStub code size by 64 bytes, and reduces CallApiCallbackStub
> by 27 bytes.
> 
> Change-Id: I46479077f61fdae6b260ca28c67df3509dee5c1a
> Reviewed-on: https://chromium-review.googlesource.com/963652
> Commit-Queue: Kanghua Yu <kanghua.yu@intel.com>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52005}

TBR=bmeurer@chromium.org,kanghua.yu@intel.com
Bug: chromium:827854

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ief0905a7bdba86f7b9fd20888eecdfd58bc6ea7d
Reviewed-on: https://chromium-review.googlesource.com/991712Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52314}