Some confusion has crept in over time, specifically around
the distinction between an offset to an external reference's
address and an offset to its entry in the external reference
table. This CL unifies naming and interfaces.

Drive-by: Fix formatting in macro-assembler-x64.

Bug: v8:6666
Change-Id: Iade98ca28a7304aba0254b92b553343826a08e41
Reviewed-on: https://chromium-review.googlesource.com/c/1286674
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56741}