target-ppc: implement xsabsqp/xsnabsqp instruction

xsabsqp:  VSX Scalar Absolute Quad-Precision
xsnabsqp: VSX Scalar Negative Absolute Quad-Precision

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
