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>

target/microblaze/op_helper.c

@@ -65,7 +65,7 @@ uint32_t helper_get(uint32_t id, uint32_t ctrl)
 
 void helper_raise_exception(CPUMBState *env, uint32_t index)
 {
-    CPUState *cs = CPU(mb_env_get_cpu(env));
+    CPUState *cs = env_cpu(env);
 
     cs->exception_index = index;
     cpu_loop_exit(cs);