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Aarch64 support

This change contains the necessary modifications to have Skia build and
run properly on an ARMv8 processor in aarch64 execution state.

Here's a list of the changes:

 - add an arm64 target to the build system + SK_CPU_ARM64 flag

 - MatrixTest was failing when built in Release mode. Fused MAC
   instructions were generated which made some intermediate results
   more accurate. As the test relies on result comparison, the more
   precise results when compared to others led to a gap bigger than
   what was tolerated. As I don't know if some actual skia code relies
   on results being comparable, I've disabled fused MAC instruction
   with -ffp-contract=off for arm64.

 - Modify include/core/SkOnce.h to have barriers work.

 - SK_CPU_ARM64 implies SK_ARM_NEON_MODE_ALWAYS.

 - use existing Xfermode optimisations with modifications that can be
   removed in the future when toolchains are ready. Also save a few
   instructions is two Xfermodes (will apply to ARM too).

 - use existing SkBoxBlur and SkMorphology optimisations.

 - use existing SkBlitMask optimisations

 - use existing BitmapProcState and Convolution optimisations.

Future changes will include:

 - Blitters (only partialy merged upstream)

 - SkUtils (there's little value in sending asm optimisations without
   having them benchmarked on real hardware).
Signed-off-by: Kevin PETIT <kevin.petit@arm.com>

BUG=skia:

Committed: http://code.google.com/p/skia/source/detail?r=13980

R=djsollen@google.com, reed@google.com, mtklein@google.com, halcanary@google.com

Author: kevin.petit@arm.com

Review URL: https://codereview.chromium.org/143423004

git-svn-id: http://skia.googlecode.com/svn/trunk@14025 2bbb7eff-a529-9590-31e7-b0007b416f81