GYP groudwork for half-float opts support.

GYP groudwork for half-float opts support.

This sets us up two new opts targets with the immediate goal of adding half-float (SkHalf.h) opts:
  - opts_neon_fp16: uses hardware support on most ARM chips with NEON to do 4 conversions at a time;
  - opts_avx: uses hardware support on Intel chips with AVX to do 8 conversions at a time.

opts_avx will be a handy thing to have around later too, especially if we want to work with floats.

This doesn't actually add any new source files to these libraries yet, so they're no-ops for now.
I'll need to write a parallel change to Chrome's GN and GYPs before we can start adding sources.

This also rolls GYP up to head, to get suppport for EnableEnhancedInstructionSet: '3' on Windows,
which is how we turn on AVX there.  There's no Mac-specific flag, so we use OTHER_CPLUSPLUSFLAGS.

BUG=skia:

TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/46b80833394d7919cadf2abf2b93802141dd21c5

[mtklein_C, 2015-02-10 17:07:08]

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

[mtklein, 2015-02-10 17:09:50]

mtklein@google.com changed reviewers:
+ mtklein@google.com

[mtklein, 2015-02-10 17:09:50]

Landing this TBR so I can get working on the Chromium-side changes after it
rolls.

[mtklein, 2015-02-10 17:09:53]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-02-10 17:10:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/915693002/1

[commit-bot: I haz the power, 2015-02-10 17:17:10]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/46b80833394d7919cadf2abf2b93802141dd21c5

[reed1, 2015-02-10 17:23:12]

lgtm

[mtklein, 2015-02-11 02:17:33]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/912223002/ by mtklein@google.com.

The reason for reverting is: Going to punt on 16-bit float support for now. 
Can't figure out ARM 64..