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/ppc/cpu.h

@@ -1203,7 +1203,6 @@ struct PowerPCCPU {
     int32_t mig_slb_nr;
 };
 
-#define ENV_OFFSET offsetof(PowerPCCPU, env)
 
 PowerPCCPUClass *ppc_cpu_class_by_pvr(uint32_t pvr);
 PowerPCCPUClass *ppc_cpu_class_by_pvr_mask(uint32_t pvr);