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.
Actually, this function is a bit weird: it returns -errno for failure, 1 for success and 0 for other types of failure. Since it's called `uv__file_is_a_directory` i'd go for returning 1 on sucess and 0 on failure, swallowing the error.