target-ppc: Add VSX ISA2.06 xadd/xsub Instructions

This patch adds the floating point addition and subtraction
instructions defined by V2.06 of the PowerPC ISA: xssubdp,
xvsubdp and xvsubsp.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
