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/tilegx/cpu.h

@@ -138,7 +138,6 @@ typedef struct TileGXCPU {
     CPUTLGState env;
 } TileGXCPU;
 
-#define ENV_OFFSET offsetof(TileGXCPU, env)
 
 /* TILE-Gx memory attributes */
 #define MMU_USER_IDX    0  /* Current memory operation is in user mode */