add API to set rpath for libpython from build scripts

[davidhewitt]

Possible replacement for #5622

This PR adds pyo3_build_config::add_libpython_rpath_link_args(), to set rpath to libpython from crate build scripts.

The idea here is that using rpath should make it trivial to link to the correct libpython; we already have add_python_framework_link_args which is doing the same thing (and should probably merge with this proposed function).

I remove pyo3-ffi-check's use of the extension-module feature and use this API instead, which should demonstrate if it works...