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 error is when the fixed size buffer being used to store the environment variable PATH is not big enough to handle the path.

The asserts are indicators of programming errors - they are checking the size passed into this function. that value really should be PATH_MAX but can be any value large enough to store the fully qualified path name to the executable.