elfcopy: Handle objects without a ".shstrtab" section string table

As of LLVM revision 238073, LLVM stores symbols and section names in
the same string table. From the upstream commit mesage:

With the scheme of naming sections like ".text.foo" where foo is a
symbol, there is a big potential saving in using a single one.

This is a cherry-pick of ELF Tool Chain revision 3225.

Sponsored by: The FreeBSD Foundation