fix(aix): add aix directory synchronization support

[pmur]

AIX doesn't support a proper flock like linux, but it seems to have enough support for process level file locking using fcntl.

For #2035

Problem

GOOS=aix does not build. AIX does not support a linux-like flock, and does not export unix.Flock.

Solution

Create an AIX specific directory-locking implementation using the AIX version of flock.