This CL adds back the 8-byte element types and extends the fast API
by hiding the unaligned memory reads performed for them.

Bug: chromium:1052746
Change-Id: Ide49ce6bd2c77b9d2d544ca2df47b5f95c93eaa9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3056988
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76207}