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...

[davidhewitt]

Anyone got concerns with this? Otherwise I'll merge to unblock #5588

[Icxolu]

Sounds reasonable to me. I assume this flag is simply ignored on windows, as I don't think there is an equivalent to rpath there. Or should we gate it behind an not(windows) anyway?

[davidhewitt]

It seems like it just works: https://github.com/PyO3/pyo3/actions/runs/19465508572/job/55701947465 - built and ran ffi check fine on Windows.

If it turns out to be a problem later, we can internally make it a noop on windows, which will be easier for users to consume anyway.