PA: Compute bucket order constants

This CL introduces no behavioral change. It changes the hardcoded
C-style arrays of bucket

*   order index shifts and
*   order sub-index masks

into templated, computed `std::array`s. As a side effect, the
preprocessor scaffolding is stricken.

Change-Id: I38bf798f0619cf6c536846ae636220e4ad0d102b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5400858


Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1280097}