Create a helper wasm-simd-utils to consolidate common helpers shared
between simd and relaxed-simd.

Drive-by cleanup to move RoundingAverageUnsigned out from
overflowing-math (there is nothing overflowing about it).

Bug: v8:11583
Change-Id: I9e24b4c1ee7f0bc00d0a3f85e7553991007a8d5a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773784Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73582}