Make it consistent so that registers in all architecture have a member
function called `bit()`.

Bug: 
Change-Id: Ie6323f81d4ecab1557259a43a30100d8da8b35f1
Reviewed-on: https://chromium-review.googlesource.com/618872
Commit-Queue: Albert Mingkun Yang <albertnetymk@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47401}