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.
I have no experience with this, but according to http://www-01.ibm.com/support/knowledgecenter/ssw_aix_53/com.ibm.aix.basetechref/doc/basetrf1/mntctl.htm%23isl60marc

"If the Size parameter indicates the supplied buffer is too small to hold the vmount structures for all the current VFSs, the mntctl subroutine sets the first word of the Buffer parameter to the required size (in bytes) and returns the value 0"

So, can we use that instead of the infinite loop and the malloc multiplier?