add stub for __shfl_xor, so can put back Half.h in eigen
add stub for __shfl_xor, so can put back Half.h in eigen

hughperkins/coriander,hughperkins/coriander,hughperkins/cuda-on-cl,hughperkins/cuda-on-cl,hughperkins/cuda-on-cl,hughperkins/cuda-on-cl,hughperkins/coriander,hughperkins/coriander