[ELF] Add ELFOSABI_GNU.

ELFOSABI_LINUX is a historical alias for ELFOSABI_GNU according to
<http://www.sco.com/developers/gabi/latest/ch4.eheader.html>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183339 91177308-0d34-0410-b5e6-96231b3b80d8