Add a --libplatdir config option for py3.9
Downstream packagers want to be able to change the layout of the packaged PyPy. CPython allows passing in arguments to
./configure (which is not used on windows). The parallel for PyPy would be a translation command line argument. The argument needs to modify
sys.libplatdir which is currently hardcoded to "lib"
. This could replace some of the [lib64` patches](https://github.com/fedora-python/cpython/pull/1/files) (all the patches for cpython3.8) used in Fedora.