ppce500_spin: Replace open-coded CPU loop with qemu_get_cpu()

Potentially env could be NULL whereas cpu would still be valid and
correspond to a previous env.

Wrapping this in qemu_get_cpu(), env is no longer needed, so simplify
code that existed before 55e5c2850293547203874098f7cec148ffd12dfa.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
