target/m68k: Use env_cpu

target/m68k: Use env_cpu

Cleanup in the boilerplate that each target must define.
The combination CPU(m68k_env_get_cpu) should have used
ENV_GET_CPU to begin; use env_cpu now.

Acked-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>