build: add /opt/freeware/... to AIX library path

To ease the use of the AIX binaries, add
/opt/freeware/lib/pthread{/ppc64} into the search path encoded into the
library, so that any version the user has installed from the common
download locations will work out of the box without having to explicitly
set LIBPATH in their environment.

PR-URL: https://github.com/nodejs/node/pull/10128


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>