This bug bites quite often when debootstrapping older Debian and/or Ubuntu releases when trying to create chroot for testing backwards compatibility of whatever software. It's very annoying.
Of course, either uname26 or setarch will work but both of them lack the ability to specify a custom version string for uname. The generated version is always 2.6.x which will - in some cases - go to a completely different branch in a (buggy) libc6 preinst script.
And eglibc is most likely not the only package now or in the future with problems like this.
Would strongly recommend fixing this somehow, if nothing else, then incorporating the use of setarch with appropriate, additional command line option. It could be refined later if and when situation occurs.
Regards,
Jussi