clangcl use compare_win for 32 bit, allowing fallback and enabling avx2 code for clang.
move defines/protos to compare_row.h
fix issue with odd width ARGBCopyAlpha functions by copying destination to temp buffer, then doing alpha copy, then copy back to destination.

R=harryjin@google.com
TBR=harryjin@google.com
BUG=libyuv:484

Review URL: https://webrtc-codereview.appspot.com/59379004.