jumper, implement 2.2 stages with approx_powf

jumper, implement 2.2 stages with approx_powf

My main interest is getting rid of weird code, but it's also faster.
The new bench drops from 667 to 412.

Change-Id: Ibf889601284cf925780320c828394f79937dc705
Reviewed-on: https://skia-review.googlesource.com/14035
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>