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/hppa/cpu.h

@@ -222,7 +222,6 @@ struct HPPACPU {
     QEMUTimer *alarm_timer;
 };
 
-#define ENV_OFFSET      offsetof(HPPACPU, env)
 
 typedef CPUHPPAState CPUArchState;
 typedef HPPACPU ArchCPU;