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.

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[pmur]

Hi, is there something I need to fix in the CL, or is this a CI issue?