Further fix extended alignment for older g++.

Further fix extended alignment for older g++.

Commit 6ceef9408 was still one brick shy of a load, because it caused
any usage at all of PGIOAlignedBlock or PGAlignedXLogBlock to fail
under older g++.  Notably, this broke "headerscheck --cplusplus".
We can permit references to these structs as abstract structs though;
only actual declaration of such a variable needs to be forbidden.

Discussion: https://www.postgresql.org/message-id/3119480.1769189606@sss.pgh.pa.us