When building a shared library with exports.txt there's no need to build an
intermediary static library, we can just pass -Wl,-bE:... when generating the
.so.
When building a shared library without exports.txt, there's no need to call
mkldexport.sh to export all symbols, because all symbols are exported anyway,
and we don't need the export file on the import side (like we do for
postgres.imp).
This makes building .so's on aix a lot more similar to building on other
platforms. In particular, we don't create and remove a .a of the same name but
different contents anymore.
Discussion: https://postgr.es/m/20220820174213.d574qde4ptwdzoqz@awork3.anarazel.de
fe6a64a58a