target/sh4: Use env_cpu, env_archcpu

target/sh4: Use env_cpu, env_archcpu

Cleanup in the boilerplate that each target must define.
Replace sh_env_get_cpu with env_archcpu.  The combination
CPU(sh_env_get_cpu) should have used ENV_GET_CPU to begin;
use env_cpu now.

Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>