Reland "remove loop in powf"

This is a reland of b23d7758dc8a8bba50122168a3a286ba0b64994c

This time, guarded by SKCMS_LEGACY_POWF, and add a hook into our
BUILD.gn to let us set defines for skcms in chromium/src/skia/BUILD.gn.

Original change's description:
> remove loop in powf
>
> Looking at the diff, I don't think we'd land the CL in the other direction.
>
> Bug: chromium:842374
>
> Change-Id: I9cbfbcbabbe6a3b36c1369e1ab81ff953cfb3c0b
> Reviewed-on: https://skia-review.googlesource.com/128004
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> Auto-Submit: Mike Klein <mtklein@chromium.org>

Bug: chromium:842374
Change-Id: I259d6072bab8008847177d417e397a148dc697d7
Reviewed-on: https://skia-review.googlesource.com/128041
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
38 files changed