Work around bug in xgetbv for Visual Studio.

xgetbv is generating bad code, falsely disabling AVX2 and AVX512.
disable optimization for the function affected on older versions of Visual C 32 bit.

R=brucedawson@chromium.org, dhrosa@google.com, harryjin@google.com
BUG=libyuv:529

Review URL: https://codereview.chromium.org/1503393004 .