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/xtensa/cpu.h

@@ -559,8 +559,6 @@ struct XtensaCPU {
     CPUXtensaState env;
 };
 
-#define ENV_OFFSET offsetof(XtensaCPU, env)
-
 
 bool xtensa_cpu_tlb_fill(CPUState *cs, vaddr address, int size,
                          MMUAccessType access_type, int mmu_idx,