Atomics overhaul.

Atomics overhaul.

This merges and refactors SkAtomics.h and SkBarriers.h into SkAtomics.h and
some ports/ implementations. The major new feature is that we can express
memory orders explicitly rather than only through comments.

The porting layer is reduced to four template functions:
  - sk_atomic_load
  - sk_atomic_store
  - sk_atomic_fetch_add
  - sk_atomic_compare_exchange
From those four we can reconstruct all our previous sk_atomic_foo.

There are three ports:
  - SkAtomics_std:    uses C++11 <atomic>,             used with MSVC
  - SkAtomics_atomic: uses newer GCC/Clang intrinsics, used on not-MSVC where possible
  - SkAtomics_sync:   uses older GCC/Clang intrinsics, used where SkAtomics_atomic not supported

No public API changes.
TBR=reed@google.com

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/a669bc7a7ae7580c5cd92067aeb95d09e64ea720

[mtklein, 2015-02-02 19:37:04]

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

[bungeman-skia, 2015-02-02 20:14:35]

lgtm, so far as any atomics thing lgtm

[mtklein, 2015-02-02 20:15:08]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-02-02 20:15:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/896553002/80001

[commit-bot: I haz the power, 2015-02-02 20:22:10]

Committed patchset #5 (id:80001) as
https://skia.googlesource.com/skia/+/a669bc7a7ae7580c5cd92067aeb95d09e64ea720