This is the last remaining missing instruction from the MVP. This CL
adds support for ia32, x64, arm, and arm64.
For CPUs which do not support the POPCNT instruction, there exists a
fallback implementation in C.

R=jkummerow@chromium.org

Bug: v8:9919
Change-Id: Ie7a79a46e91726e15379b9a21b59775bbf5de556
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895569
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64764}