Add sponsor button and link to liberapay

This is not really an issue per se, but apart from https://github.com/bjorn3/rustc_codegen_cranelift/issues/1090 there was no mention of it.

From reading https://www.reddit.com/r/rust/comments/15drwt8/progress_report_on_rustc_codegen_cranelift_july/ it seems people don't know they can sponsor you/this project via liberapay.

Github allows to show a "Sponsor" button at the top of the repository, and it can link to your https://liberapay.com/bjorn3/. I've tried to do it via a PR but it of course it's something that needs the repo owner to change its settings.

The steps are described here https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository#displaying-a-sponsor-button-in-your-repository

• Change the repository general settings to display the sponsorship link, that will open an editor to create the .github/FUNDING.yml file
• the contents should be something like liberapay: bjorn3
• validating will commit the file and that should be the end of it

Use the repository settings flow described in the issue to enable the Sponsor button and create .github/FUNDING.yml. Set the Liberapay entry to bjorn3, then verify that the repository displays the sponsorship link and that the funding file is committed.