bpo-43666: Lib/_aix_support.py routines may fail in a WPAR environment (GH-25095)

Since WPAR and LPAR both have a builddate for teh fileset bos.rte
The name of the fileset checked is modified.
To prevent a similiar situation (no builddate in ODM) a value
sufficient for pep425 activity if retrieved buildate is zero or NULL
Patch by M Felt.