Log Message:
python: Override sys.platform with PY_PLATNAME when cross-compiling.

For this purpose, factor the PY_PLATNAME definition out into a new
lang/pythonNN/platname.mk file.  It's not the same for 2.x and 3.x;
perhaps we could factor it out further into a single 2.x vs 3.x
conditional but this is a more mechanical change that makes it easier
to audit for now.