[yaml2obj][ELF] Ensure more fields are zero'd.

I was spotting garbage in the output. I'd like to just zero the entire
ELFYAML::Section to be sure, but it contains non-POD types. (I'm also
trying to avoid bloating the ELFYAML::Foo classes with a bunch of
constructor code).

No test, since this is by its very nature unpredictable. I'm pretty sure
that one of the sanitizers would catch it immediately though.
[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.
[yaml2obj][ELF] Factor out string table section creation.