Cleanup in the boilerplate that each target must define.
Replace mips_env_get_cpu with env_archcpu. The combination
CPU(mips_env_get_cpu) should have used ENV_GET_CPU to begin;
use env_cpu now.
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>