Well a few spare time let me investigate this way: using gc[cj] libffi.

I so first install libffi-4.2.0-3 as well as libffi-devel-4.2.0-3 (not
sure this last one is required, though) then configure python (2.6) with
--with-system-ffi but it doesn't seems to be enough to locate libffi so
I added symbolic links:
/usr/lib/libffi.a -> ../../opt/freeware/lib/libffi.a

/usr/include/ffi.h -> \
../../opt/freeware/lib/gcc/powerpc-ibm-aix5.3.0.0/4.2.0/include/ffi.h
/usr/include/ffitarget.h -> \
../../opt/freeware/lib/gcc/powerpc-ibm-aix5.3.0.0/4.2.0/include/ffitarget.h

After that I reach to compile python2.6.2 without error and at a first
glance it seems to work fine (just rebuild bzr 1.17 and launch some
cmdl, ;<)).

Hope this could help for python3 and xlC,
    r.