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/arm/cpu.h

@@ -917,8 +917,6 @@ void arm_cpu_post_init(Object *obj);
 
 uint64_t arm_cpu_mp_affinity(int idx, uint8_t clustersz);
 
-#define ENV_OFFSET offsetof(ARMCPU, env)
-
 #ifndef CONFIG_USER_ONLY
 extern const struct VMStateDescription vmstate_arm_cpu;
 #endif