python: improved cross-compiling avoiding host libs to leak in and getting linker errors
python: add patch to not precompile .pyc and .pyo files when cross-compiling
python: add patch to use the pgen we compiled at stage 0 and which can be run on the build host when cross-compiling
python: changed to install the pgen we need at cross stage and which runs on the build host

We inject the `pgen` built at toolchain stage as `python-pgen` which we use
at cross stage (see: use-hostpgen.patch.cross).
python: fixed linking of modules when cross-compiling

When linking modules libpython2.7.so is not installed in the sandbox but
available in the top-level source directory.