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>

include/exec/gen-icount.h

@@ -5,6 +5,8 @@
 
 /* Helpers for instruction counting code generation.  */
 
+#define ENV_OFFSET   offsetof(ArchCPU, env)
+
 static TCGOp *icount_start_insn;
 
 static inline void gen_tb_start(TranslationBlock *tb)