Sk2x for NEON

Sk2x for NEON

Also decreases the precision of Sk4f::rsqrt() for speed, keeping Sk4f::sqrt() the same:
instead of doing two estimation steps in rsqrt(), do one there and one more in sqrt().

Tests pass on my Nexus 7.  float64x2_t is still a TODO for when I get a hold of a Nexus 9.


BUG=skia:

Committed: https://skia.googlesource.com/skia/+/5553bedf603e1f1a55d2fd6c70c2ca3a831b3801

[mtklein_C, 2015-03-19 18:41:12]

mtklein@chromium.org changed reviewers:
+ msarett@google.com

[mtklein_C, 2015-03-19 18:41:17]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-03-19 18:41:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1018423003/1

[commit-bot: I haz the power, 2015-03-19 18:41:27]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-03-20 00:41 UTC

[msarett, 2015-03-19 18:43:16]

lgtm

[mtklein, 2015-03-19 18:54:14]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-19 18:54:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1018423003/20001

[commit-bot: I haz the power, 2015-03-19 19:08:33]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/5553bedf603e1f1a55d2fd6c70c2ca3a831b3801