I have the osso-software-version-rx*4-unlocked package, so it didn't cause any problems upgrading libc6, and then the System Upgrade was able to work straight up from Application Manager. If you currently have the regular osso-software-version-rx*4 package installed, try removing it, and then see if you can then upgrade libc6 and the system. I know this can be a bit dangerous, but if you want the system upgrade, and don't want to uninstall libc6-dev and lose your build environment, this may be the only choice. It worked for me.