SkXbyak basics

SkXbyak basics

A little JIT proof of concept for SkRasterPipeline, using xbyak, which is a header-only assembler.  It's x86-only, but supports x86 very thoroughly, and it's very user friendly (at least as far as assembler libraries go...).

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: Ie17e562b0f3fff5914041badfb2c1fe4f86efab8
Reviewed-on: https://skia-review.googlesource.com/5730
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>