The source of random numbers in Ubuntu come from the Linux kernel, specifically /dev/random and /dev/urandom (pseudo-random). The difference between the two is that /dev/random may be blocking while /dev/urandom does not but may not be as random.

That aside, RdRand is just one of many sources of entropy used in the Linux kernel and as long as userspace applications don't use RdRand directly but make a system call to the kernel, there is no issue with Ubuntu's random output.

Here is a link to Slashdot story where Linus explained why a petition removing RdRand from the kernel would be stupid