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/mips/cpu.h

@@ -1071,7 +1071,6 @@ struct MIPSCPU {
     CPUMIPSState env;
 };
 
-#define ENV_OFFSET offsetof(MIPSCPU, env)
 
 void mips_cpu_list(void);