It was *probably* eglibc 2_15, before eglibc was merged back into glibc.
I'd highly recommend doing that with a real rootfs-building tool, like buildroot or something else, because, err, it'll make your life much, much, much easier. Plus, *you* will be dictating the TC, and the usual rootbuilding tools obviously do roll a TC as part of the process.
Things will get hairy if you want to match the Kobo's vanilla env for compatibility, because building older glibcs can be just plain hellish.
EDIT: I have no personal stake in the choice of tools, as I've never used one myself, I stick to x-compiling userland stuff with ct-ng TCs

[But since that *does* involve x-compiling older glibcs, I have a long and varied history with that ^^]. I do know people have used buildroot in the past for Kindle stuff, but that's it

.