This change includes templatization of the test helper to allow the
same function to be reused for both signed and unsigned data types.

We implement a new function RoundingAverageUnsigned in overflowing-math,
rather than in base/utils, since the addition could overflow.

SIMD scalar lowering and implementation for other backends will follow
in future patches.

Bug: v8:10039
Change-Id: I70735f7b6536f197869ef1afbccaf5649e7e8448
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1958007Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65531}