libvpx

Add sse2 versions of halfpix variance

These were lost in the great sub pixel variance move of
6a82f0d7fb9ee908c389e8d55444bbaed3d54e9c

Not having these functions caused a ~10% performance regression in
some realtime vp8 encodes.

Change-Id: I50658483d9198391806b27899f2c0d309233c4b5