[ELF] Don't compare an atom with itself in compareByPosition().

This caused some tests to fail on FreeBSD, and Mac OS X.
Some std::sort() implementations will check for strict-weak-ordering
by comparing with the same element, or will compare an element to
itself for 1-element sequence. Take care of this case. Thanks to
chandlerc for explaning that to me.

Reviewed by:	ruiu

llvm-svn: 227709