Prototype i32x4.extadd_pairwise_i16x8_{s,u} and
i16x8.extadd_pairwise_i8x16{s,u} (names not confirmed) on ARM64 and
interpreter. With a simple test case.

Bug: v8:11086
Change-Id: If1ffc04e179e86ca5cc209bf9ef9d337298e3cc2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2513872Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71068}