target-ppc: Fix xscmpodp and xscmpudp instructions

- xscmpodp & xscmpudp are missing flags reset.
- In xscmpodp, VXCC should be set only if VE is 0 for signalling NaN case
  and VXCC should be set by explicitly checking for quiet NaN case.
- Comparison is being done only if the operands are not NaNs. However as
  per ISA, it should be done even when operands are NaNs.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
