Add support for win64 flags to yasm.

This change is needed to add win64 support to libvpx.

BUG=166496, 170672


Review URL: https://chromiumcodereview.appspot.com/11971037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@177653 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed