target-ppc: VSX Stage 4: Add xssqrtsp

This patch adds the VSX Scalar Square Root Single Precision (xssqrtsp)
instruction.

The existing VSX_SQRT() macro is modified to support rounding of the
intermediate double-precision 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>
