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/nios2/cpu.h

@@ -194,7 +194,6 @@ typedef struct Nios2CPU {
     uint32_t fast_tlb_miss_addr;
 } Nios2CPU;
 
-#define ENV_OFFSET offsetof(Nios2CPU, env)
 
 void nios2_tcg_init(void);
 void nios2_cpu_do_interrupt(CPUState *cs);