Support reading GNU symbol versions in ELFObjectFile

* Add enums and structures for GNU version information.
* Implement extraction of that information on a per-symbol basis (ELFObjectFile::getSymbolVersion).
* Implement a generic interface, GetELFSymbolVersion(), for getting the symbol version from the ObjectFile (hides the templating).
* Have llvm-readobj print out the version, when available.
* Add a test for the new feature: readobj-elf-versioning.test

llvm-svn: 152436