target/microblaze: Use env_cpu, env_archcpu

target/microblaze: Use env_cpu, env_archcpu

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

Move cpu_mmu_index below the include of "exec/cpu-all.h",
so that the definition of env_archcpu is available.

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>