Runtime CPU detection for rsqrt().

Runtime CPU detection for rsqrt().

This enables the NEON sk_float_rsqrt() code for configurations that have NEON at run-time but not compile-time.

These devices will see about a 2x (1.26 -> 2.33) slowdown in sk_float_rsqrt(), but it should be more precise than our portable fallback.

(When inlined, the portable fallback and the NEON code are almost identical in speed.  The only difference is precision.  Going through a function pointer is causing all this slowdown.  This is a good example of a place where Skia really benefits from compile-time NEON.)

BUG=skia:4117, skia:4114
No public API changes.
TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/f684a78d9ea988883c9b2c7bcc4ea4d5e68bd998

[mtklein_C, 2015-07-30 15:37:35]

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

[commit-bot: I haz the power, 2015-07-30 15:37:40]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1264893002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1264893002/1

[mtklein_C, 2015-07-30 15:44:35]

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

[commit-bot: I haz the power, 2015-07-30 15:45:10]

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

[commit-bot: I haz the power, 2015-07-30 15:45:10]

Dry run: This issue passed the CQ dry run.

[jvanverth1, 2015-07-30 16:28:35]

lgtm

[mtklein, 2015-07-30 16:28:55]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-07-30 16:29:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1264893002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1264893002/1

[commit-bot: I haz the power, 2015-07-30 16:29:41]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f684a78d9ea988883c9b2c7bcc4ea4d5e68bd998