target/ppc: Use env_cpu for cpu_abort in excp_helper

Use the env_cpu function to get the CPUState for cpu_abort. These are
only needed in case of fatal errors so this allows to avoid casting
and storing CPUState in a local variable wnen not needed.

Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>