Splits the 64bit operation to a seperate function since there are
different return types depending upon whether the architecture is
64-bit or 32-bit.

BUG=v8:6949,v8:11074

Change-Id: If196cf658298ca0a1e5a13e1db812178307e7d12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531789
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71132}