I'm not sure how this can work in terms of license.
The kernel is GPL2, and userspace code is not seen as being a derivative work. So libc can have whatever license it wants. But if the two are worked on as a single project, presumably that means libc would need to be GPL2 as well.
For one thing, that limits the libcs that can be used here. I believe LGPL code can be relicensed as GPL, so that might work for an LGPL libc, but only if the libc is not GPL3+ (not sure what GNU libc is). I guess a permissively licensed libc (e.g. newlib) would be ok.
However, the bigger matter is what this would mean for other userspace code - can they link against a GPLed libc if they are not GPLed? That was the whole reason for using the LGPL instead of the GPL for GNU libc, I believe?