AIX: Link the postgres executable with -Wl,-brtllib.

This allows PostgreSQL modules and their dependencies to have undefined
symbols, resolved at runtime.  Perl module shared objects rely on that
in Perl 5.8.0 and later.  This fixes the crash when PL/PerlU loads such
modules, as the hstore_plperl test suite does.  Module authors can link
using -Wl,-G to permit undefined symbols; by default, linking will fail
as it has.  Back-patch to 9.0 (all supported versions).