mingw gcc sets the macro _M_IX86 which is normally only set
by Visual C and clangcl which are Visual C style source code
style for assembly, but gcc is not Visual C compatible.
Add _MSC_VER to most ifdefs to detect that its really Visual C
or clangcl and not mingw gcc so the gcc source code will be used.

Bug: libyuv:744
Test: CXXFLAGS=-m32 CXX=~/prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/bin/x86_64-w64-mingw32-g++ make -f linux.mk
Change-Id: I3431aa486eb769b145faa8d5eb75ed639f9d6f5e
Reviewed-on: https://chromium-review.googlesource.com/722319Reviewed-by: Cheng Wang <wangcheng@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>