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.