Reorg Sk4x to match the pattern of SkPMFloat.

Reorg Sk4x to match the pattern of SkPMFloat.

No real changes here, just moving files around:
  - move impl files into src/opts
  - rename _portable _none

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/f87cbcdf9f10120ca1821b4d6de94f7a2e94f930

[mtklein_C, 2015-03-19 15:06:07]

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

[mtklein, 2015-03-19 16:47:04]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-19 16:47:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1021713004/1

[commit-bot: I haz the power, 2015-03-19 16:47:16]

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 2015-03-19 22:47 UTC

[reed1, 2015-03-19 17:17:43]

lgtm

[commit-bot: I haz the power, 2015-03-19 17:19:52]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f87cbcdf9f10120ca1821b4d6de94f7a2e94f930