target-ppc: Fix and enable fri[mnpz]

The fri* series of instructions was introduced prior to ISA 2.06 and
is supported on Power7 and Power8 hardware.  However, the instruction
is still considered illegal in the P7 and P8 QEMU emulation models.
This patch enables these instructions for the P7 and P8 machines.

Also, the existing helper is modified to correctly handle some of
the boundary cases (NaNs and the inexact flag).

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