cpu: Add CPUArchState pointer to CPUState

The target-specific ENV_GET_CPU() macros have allowed us to navigate
from CPUArchState to CPUState. The reverse direction was not supported.
Avoid introducing CPU_GET_ENV() macros by initializing an untyped
pointer that is initialized in derived instance_init functions.

The field may not be called "env" due to it being poisoned.

Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>
