row.h adds CLANG_HAS_AVX512
function ifdefs in row.h for avx512
source code ifdefed function by function for
avx512 and avx2.

Bug: libyuv:778
Test: LibYUVConvertTest.NV21ToRGB24_Opt
Change-Id: If32b51459685d0d5785c5c1e94c8f668f8e74b55
Reviewed-on: https://chromium-review.googlesource.com/982402
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: richard winterton <rrwinterton@gmail.com>