Add predefined version identifiers for ELFv1 and ELFv2.

For PPC64 LittleEndian a new ELF version was defined. While the common usage is
ELFv1 with big endian and ELFv2 with little endian, the other uses are also possible.
In fact gcc defines a command line switch --with-abi=elfv1 / --with-abi=elfv2 to choose
the ELF ABI.
The new predefined version identifiers ELFv1 / ELFv2 reflect the fact that this
information cannot be determined elsewhere.
See http://llvm.org/devmtg/2014-04/PDFs/Talks/Euro-LLVM-2014-Weigand.pdf for more
information on ELFv2.
Merge pull request #4096 from redstar/elf

Add predefined version identifiers for ELFv1 and ELFv2.