build: add shared library support to AIX build

Updates to build the shared library version of node on AIX. Adds the
same functionality to AIX that was added on Linux under this:

Ref: https://github.com/nodejs/node/pull/6994/

PR-URL: https://github.com/nodejs/node/pull/9675


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>