Fix building with clang targeting x86_32 mingw
Fix building with clang targeting x86_32 mingw

In these cases, CLANG_HAS_AVX2 is defined, but _MSC_VER isn't.

The corresponding codepaths are in source/compare_win.cc, and the whole
file is guarded with a defined(_MSC_VER).

This is the same fix as in d8680893ecfcffd952c7a5d0dc1878d1c486a4e4,
but this issue wasn't noticed since compare.o wasn't linked in in
earlier tested configurations.

Change-Id: Ifff6d4bd7b1071256df1169881490a52026c183b
Reviewed-on: https://chromium-review.googlesource.com/988573
Reviewed-by: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@chromium.org>
Commit-Queue: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@chromium.org>

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv