After JDK-8354686 the ubsan vptr checks still did not work. We finally found out, that they only work, if all linkage units are linked with the C++ compiler frontend as linker, especially the main executables (java, javac, ...) which are linked with the C compiler frontend.
So for a ubsan enabled build on AIX we let everything link with the C++ compiler frontend.
Background: The C++ compiler frontend inherently adds special static libraries which the C compiler does not. This results in missing symbols when trying to start a program and its shared libraries, when they were linked in mixed mode.