Basically you don't have a "libc.so.6" file and mkinitrd wants one.
Look in your /lib directory and you will notice that you don't a "libc.so.6" file. Why? Who knows...
RPMfind is a great tool here, looking up your RPM that you want to install brings us to this page:
Where it lists what the RPM provides, and what it requires. Notice that you have quite a few libc.so.6 libraries that you can choose from.
Following the link for libc.so.6(GLIBC_2.4) brings you to:
Beware! Changing your libc's is a lot like playing russian roulette with a semi-automatic. Okay, so you only had one bullet in the clip...
It's a great way to get an unstable, unsecure machine. Be careful.