Splits the 64bit operations 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: I13cc576a26f60288281c42df3326ba902fd36dbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2529910
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71120}