cpu: Change tcg_cpu_exec() arg to cpu, not env

cpu: Change tcg_cpu_exec() arg to cpu, not env

The sole caller of this function navigates the cpu->env_ptr only for
this function to take it back the cpu pointer straight away. Pass in
cpu pointer instead and grab the env pointer locally in the function.
Removes a core code usage of ENV_GET_CPU().

Reviewed-by: Andreas Färber <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Signed-off-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>