[assembler] Make register definitions constexpr

[assembler] Make register definitions constexpr

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, and even recommends to
change existing code. 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

[Clemens Hammacher, 2017-03-03 19:04:20]

The CQ bit was checked by clemensh@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-03 19:04:24]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Clemens Hammacher, 2017-03-03 19:04:26]

clemensh@chromium.org changed reviewers:
- bmeurer@chromium.org, ishell@chromium.org, mstarzinger@chromium.org

[commit-bot: I haz the power, 2017-03-03 19:30:33]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-03 19:30:34]

Dry run: This issue passed the CQ dry run.

[Clemens Hammacher, 2017-03-06 09:24:06]

The CQ bit was checked by clemensh@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-06 09:24:07]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-06 09:51:46]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-06 09:51:47]

Dry run: This issue passed the CQ dry run.