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/microblaze/cpu.h

@@ -310,7 +310,6 @@ struct MicroBlazeCPU {
     CPUMBState env;
 };
 
-#define ENV_OFFSET offsetof(MicroBlazeCPU, env)
 
 void mb_cpu_do_interrupt(CPUState *cs);
 bool mb_cpu_exec_interrupt(CPUState *cs, int int_req);