Factor out and share implementation of the sign() calc() function

Even though it is simple, it's complicated enough that it is better to
avoid repeating it.

Change-Id: I828c9c33a9a0bc5295ee9d4c653b8e7c58cf6dc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6633480
Reviewed-by: Daniil Sakhapov <sakhapov@chromium.org>
Commit-Queue: Daniil Sakhapov <sakhapov@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/main@{#1473014}