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/xtensa/cpu.h

@@ -564,8 +564,6 @@ static inline XtensaCPU *xtensa_env_get_cpu(const CPUXtensaState *env)
     return container_of(env, XtensaCPU, env);
 }
 
-#define ENV_GET_CPU(e) CPU(xtensa_env_get_cpu(e))
-
 #define ENV_OFFSET offsetof(XtensaCPU, env)