[yaml2obj][ELF] Add dummy .strtab section.

This will be needed later for holding symbol names, due to the libObject
issue mentioned in the commit message of r184161.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184242 91177308-0d34-0410-b5e6-96231b3b80d8