[yaml2obj][ELF] Refer specifically to the section header string table.

A bug in libObject will cause it to assert() if a symbol table's string
table and the section header string table are the same section, so we
need to ensure that we emit two different string tables (among other
things). The problematic code is the hardcoded usage of ".strtab"
(`dot_strtab_sec`) for looking up symbol names in
ELFObjectFile<ELFT>::getSymbolName.

I discussed this with Michael, and he has some local improvements to the
ELF code in libObject that, among other things, should fix our handling
of this scenario.

llvm-svn: 184161