Thanks to @konradybcio and nergzd723 the iPhone 6 and other A7-A8X devices can boot (very minimal) Linux!
Here's how I did it: https://web.archive.org/web/20200104145142/https://blog.forkwhiletrue.me/posts/an-almost-fully-libre-galaxy-s3/.
The prebuilt binaries weren't available anymore so I had to build U-Boot and TWRP with a patch myself, but that wasn't too hard. I've put my binaries here: https://github.com/ungeskriptet/miscellaneous