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>

linux-user/nios2/cpu_loop.c

@@ -23,7 +23,7 @@
 
 void cpu_loop(CPUNios2State *env)
 {
-    CPUState *cs = ENV_GET_CPU(env);
+    CPUState *cs = env_cpu(env);
     Nios2CPU *cpu = NIOS2_CPU(cs);
     target_siginfo_t info;
     int trapnr, ret;