GN: turn on easy libjpeg-turbo SIMD backends (ARMv7 and ARMv8)

GN: turn on easy libjpeg-turbo SIMD backends (ARMv7 and ARMv8)

These are so easy we might as well...

I did a quick check of relevant-looking defines:
   - GYP defined WITH_SIMD, but it looks like that's already defined (by jconfig.h?);
   - GYP defined RGBX_FILLER_0XFF, but that affects only x86/x86-64;
   - GYP defined STRICT_MEMORY_ACCESS, which does nothing;
   - GYP defined MOTION_JPEG_SUPPORTED, which does nothing (and we'd probably not care anyway).

BUG=skia:5875

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4745

Change-Id: Ib1f28d354630be472c4d9648d5ade74a452a9e24
Reviewed-on: https://skia-review.googlesource.com/4745
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>