target/ppc: Split out helper_dbczl for 970

We can determine at translation time whether the insn is or
is not dbczl.  We must retain a runtime check against the
HID5 register, but we can move that to a separate function
that never affects other ppc models.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>