target-ppc: VSX Stage 4: Add xsaddsp and xssubsp

This patch adds the VSX Scalar Add Single-Precision (xsaddsp) and
VSX Scalar Subtract Single-Precision (xssubsp) instructions.

The existing VSX_ADD_SUB macro is modified to support the rounding
of the (intermediate) result to single-precision.

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