Hashing should ignore the signedness of the type, since different
platforms might define standard types like {char} as either signed or
unsigned. This leads to problems if hashes are included in test
expectations, see https://crrev.com/c/1926032 and
https://crbug.com/1025184#c26.

This CL avoid such problems by always treating the input as signed
values. This also reduces binary size, since the instantiations for
int8_t and uint8_t are identical now and are folded together by the
compiler / linker.

R=jkummerow@chromium.org

Bug: chromium:1025184
Change-Id: I3fee4d8662dd1c31cd6483639fe4edd4511662c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1926769Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65090}