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/openrisc/cpu.h

@@ -317,7 +317,6 @@ typedef struct OpenRISCCPU {
 
 } OpenRISCCPU;
 
-#define ENV_OFFSET offsetof(OpenRISCCPU, env)
 
 void cpu_openrisc_list(void);
 void openrisc_cpu_do_interrupt(CPUState *cpu);