This is a reland of e2aa734a

The original change was speculatively reverted in
https://crrev.com/c/2568925 but seems unrelated to the flakey
test failures (which are on x64).

Original change's description:
> [wasm-simd][arm] Prototype i8x16.popcnt
>
> Bug: v8:11002
> Change-Id: Ib97e51ed52249a1af7a4b879396b70a016991719
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567534
> Commit-Queue: Zhi An Ng <zhin@chromium.org>
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#71552}

Bug: v8:11002
Change-Id: I714918e2640d27d29953f940e3bedeb9ea2f8a08
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567243
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71585}