Optimise sswap for POWER9
Optimise sswap for POWER9

Use lxvd2x instruction instead of lxvw4x.
lxvd2x performs far better on the new POWER architecture than lxvw4x.

kortschak/OpenBLAS,xianyi/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,kortschak/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS