target-ppc: VXSQRT Should Not Be Set for NaNs

The Power ISA square root instructions (fsqrt[s], frsqrte[s]) must
set the FPSCR[VXSQRT] flag when operating on a negative value.
However, NaNs have no sign and therefore this flag should not
be set when operating on one.

Change the order of the checks in the helper code.  Move the
SNaN-to-QNaN macro to the top of the file so that it can be
re-used.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
