Avoid literal loads in the arm vertexjit.

This should improve both armv7 and avoid the literal pool.

For 565, we're going to set all the alpha bits anyway, so might as well just clear only G.  I don't think A can have any bits bleed into it anyway.

For 5551, we only need to clear the right shifted bits from G and B.  R is safe since it goes into a wall.

-[Unknown]