The old guard only checked for defined(_M_X64) which is defined by mingw64.  Add a test for defined(_MSC_VER) which is defined for clangcl and visual c but not mingw.  mingw should use row_gcc.cc for both 32 and 64 bit.

R=harryjin@google.com
BUG=webm:1252,libyuv:613
TEST=local gcc/clang builds on linux tested and try bots for others.

Review URL: https://codereview.chromium.org/2105603002 .