[RFA] target.c: eliminate one use of DEPRECATED_REGISTER_SIZE

[Andrew Cagney]

>  default_region_size_ok_for_hw_watchpoint (int byte_count)
>  {
> -  return (byte_count <= DEPRECATED_REGISTER_SIZE);
> +  return (byte_count <= (TARGET_PTR_BIT / TARGET_CHAR_BIT));
>  }
>  
Ok, but with `TYPE_LENGTH (builtin_type_void_{code/data}_ptr)'.  Here I 
believe that the data pointer would be more correct.

Andrew