[yaml2obj] Initial ELF support.

Currently, only emitting the ELF header is supported (no sections or
segments).

The ELFYAML code organization is broadly similar to the COFFYAML code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183711 91177308-0d34-0410-b5e6-96231b3b80d8