Previously the assembly code was only available to Windows.
This CL ports the SSE2 code to GCC syntax.

When running a profiler on all the unittests, this function
was the slowest of all functions that still ran in C code.
   3.71%  libyuv_unittest  libyuv_unittest      [.] ARGBToRGB565DitherRow_C

Was
ARGBToRGB565Dither_Opt (2894 ms)
Now
ARGBToRGB565Dither_Opt (432 ms)

TBR=harryjin@google.com
BUG=libyuv:492

Review URL: https://codereview.chromium.org/1397673002 .