aix: improve AIX compatibility for Node.js use

As we work through the changes to Node required to support
PowerPC platforms, we have had to introduce a changes to the
libuv component to support the AIX operating system.

These are the initial enabling changes required.
Doing so results in the code failing to compile.

On Linux we have
```
/* Data structure describing a polling request.  */
struct pollfd
  {
    int fd;                     /* File descriptor to poll.  */
    short int events;           /* Types of events poller cares about.  */
    short int revents;          /* Types of events that actually occurred.  */
  };
```

On AIX it's different
```
#ifdef  __64BIT__
/* pollfd.fd is always a 32 bit entity (XOPEN) */
struct  pollfd
{
        int     fd;                     /* file descriptor or file ptr  */
        short   events;                 /* requested events             */
        short   revents;                /* returned events              */

};
#else /* __64BIT__ */
struct  pollfd
{
        long    fd;                     /* file descriptor or file ptr  */
        ushort  reqevents;              /* requested events             */
        ushort  rtnevents;              /* returned events              */
};
#define events  reqevents               /* SVR3,4 pollfd member name    */
#define revents rtnevents               /* SVR3,4 pollfd member name    */
#endif /* __64BIT__ */
```

The code in question here is trying to 'undo' the AIX-ism to make it more like Linux.

It's probably safe to remove the remain undef's (it appears to compile and test ok)