libgpos has a set of banned API calls which needs to be allowed with
the ALLOW_xxx macro in order for gpopt to compile (and thus run).
The changes to ereport() brought a need for allowing abort() since
it now invokes abort when building with --enable-cassert.

This is a temporary fix awaiting the removal of the banning of
function calls entirely. Pushed even though the CI pipeline failed
to provide a clean run (for seeminly unrelated reasons) due to the
absence of this blocking other efforts.