I originally needed this for the initialization of a constexpr array in
the wasm lazy compile builtin, but since it's a bigger change, I now
split it off as this separate CL.
The style guide recommends constexpr over const. I thus apply the
constexprificaton over all headers that I touched anyway.

I also remove the ARM64_DEFINE_REG_STATICS hack. It was introduced when
merging in arm64 support more than three years ago, and I don't see the
purpose for this.
Also, some #defines can now be constexpr definitions, which was not
possible before according to the comment.

R=bmeurer@chromium.org, mstarzinger@chromium.org, ishell@chromium.org

Change-Id: I6d743b4462c347d363f99e28007bc9e8c84ae617
Reviewed-on: https://chromium-review.googlesource.com/451277Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43637}