target-ppc: Implement round to odd variants of quad FP instructions

xsaddqpo:  VSX Scalar Add Quad-Precision using round to Odd
xsmulqo:   VSX Scalar Multiply Quad-Precision using round to Odd
xsdivqpo:  VSX Scalar Divide Quad-Precision using round to Odd
xscvqpdpo: VSX Scalar round & Convert Quad-Precision format to
           Double-Precision format using round to Odd
xssqrtqpo: VSX Scalar Square Root Quad-Precision using round to Odd
xssubqpo:  VSX Scalar Subtract Quad-Precision using round to Odd

In addition, fix the invalid bitmask in the instruction encoding
of xssqrtqp[o].

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
CC: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
