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.
This doesn't look like a fatal error to me, why the assert? If /aha is not there `uv_fs_event_start` should return UV_ENOSYS.