ELF: Do not mix link-once and group sections.

Previously, we put both link-once and group sections into the same map
and seaparated them out when we use them. Apparently we should put them
into seaprate maps in the first place.

This piece of code is added recently, and I still don't understand all
of them. Looks like we need to clean this up even more.

llvm-svn: 234223