python3: enable shared python3 lib; make this the norm

This will reduce the bloat when users will want to compile in their
Python C extensions.
There will be a initial bloat (several kb) if just Python
is installed, but that will be compensated when users will add more
C extensions.

During the build we also have to add Python's PKG_BUILD_DIR
so that the shared lib is found when compiling Python's
built-in C extensions.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python3: add _PYTHON_HOST_PLATFORM env var when cross-compiling C extensions

This mostly helps to avoid confusion when modules are cross-compiled.
Otherwise build folders are named with the host's platform name.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>