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>

accel/tcg/user-exec.c

@@ -680,7 +680,7 @@ static void *atomic_mmu_lookup(CPUArchState *env, target_ulong addr,
 {
     /* Enforce qemu required alignment.  */
     if (unlikely(addr & (size - 1))) {
-        cpu_loop_exit_atomic(ENV_GET_CPU(env), retaddr);
+        cpu_loop_exit_atomic(env_cpu(env), retaddr);
     }
     helper_retaddr = retaddr;
     return g2h(addr);