cpu: Move ENV_OFFSET to exec/gen-icount.h

Now that we have ArchCPU, we can define this generically,
in the one place that needs it.

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/lm32/cpu.h

@@ -195,7 +195,6 @@ struct LM32CPU {
     uint32_t features;
 };
 
-#define ENV_OFFSET offsetof(LM32CPU, env)
 
 #ifndef CONFIG_USER_ONLY
 extern const struct VMStateDescription vmstate_lm32_cpu;