target-ppc : Allow fc[tf]id[*] mnemonics for non TARGET_PPC64

This patch remove limitation for fc[tf]id[*] on 32 bits targets and
add a new insn flag for signed integer 64 conversion PPC2_FP_CVT_S64

Signed-off-by: Pierre Mallard <mallard.pierre@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
