cpu: Replace ENV_GET_CPU with env_cpu

Now that we have both ArchCPU and CPUArchState, we can define
this generically instead of via macro in each target's cpu.h.

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

target/nios2/op_helper.c

@@ -46,7 +46,7 @@ void helper_check_interrupts(CPUNios2State *env)
 
 void helper_raise_exception(CPUNios2State *env, uint32_t index)
 {
-    CPUState *cs = ENV_GET_CPU(env);
+    CPUState *cs = env_cpu(env);
     cs->exception_index = index;
     cpu_loop_exit(cs);
 }