ELF: Return TargetRelocationHandler instead of <ArcH>TargetRelocationHandler.

getRelocationHandler is a public interface to get an instance of
TargetRelocationHandler. We don't use any member function other than
applyRelocations to a returned instance. Returning a base class instance
suffices here. (If a return type is a derived class, it looks like we were
using derived classes features.)

llvm-svn: 234081