Add a CompareCharsEqual to complement CompareChars, where we only care
about equality and not ordering. For such cases, we can memcmp for two-
byte as well as one-byte strings (we can't for CompareChars because the
ordering would be incorrect on little-endian systems).

Replace uses of CompareChars that only compare the result against zero,
with CompareCharsEqual. Additionally, use some template magic to
simplify the "make unsigned" operation in these methods.

Change-Id: I0d65bee81b98d3938d15daa4af331c90558ea84f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557980
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71385}