[ELF] Do not merge sections in case of relocatable object generation

Do not merge sections if generating a relocatable object. It makes
the code simpler because we do not need to update relocations addends
to reflect changes introduced by merging. Instead of that we write
such "merge" sections into separate OutputSections and keep SHF_MERGE
/ SHF_STRINGS flags and sh_entsize value to be able to perform merging
later during a final linking.

Differential Revision: http://reviews.llvm.org/D25066

llvm-svn: 283300