The problem you should look into is why the _posixsubprocess doesn't build, the message about msvcrt is a red herring due to the way subprocess.py is structured.

Also compare the pyconfig.h files created for the two single architecture builds, maybe there are differences in detected features for the two architectures (other than CPU differences, there's a pymacconfig.h files that sets the correct values for sizes of basic types). 

That said, I won't be debugging this issue myself. MacOS on PowerPC is by now a thing of the past and IIRC the last time I had access to a 64-bit PPC machine was before GUI code would run properly in PPC64 mode.