GN: turn on libpng SSE2 filters

GN: turn on libpng SSE2 filters

I was profiling and remembered GN hasn't turned on much CPU-specific code.  I looked at zlib, then libpng, then libjpeg-turbo:

  - zlib was easy but not useful.  No routine we use in decoding changes significantly.
  - libpng was easy and useful, and we were already using NEON filters on ARM.
  - libjpeg-turbo requires yasm and is annoying.

BUG=skia:5875

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4531

Change-Id: Ie072a457da41ee6538eebacb5eb5dbe5a6eb585e
Reviewed-on: https://skia-review.googlesource.com/4531
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>