pkg-python: no longer use distutilscross for setuptools

Thanks to the bump of Python 2.x, distutilscross is no longer needed
to achieve cross-compilation for setuptools packages. The host Python
2.x interpreter can be tricked into using the target compiler thanks
to pointing it to a different sysconfigdata module, which is achieved
using PYTHON_PATH.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>