[obj2yaml] - Support dumping objects that have multiple SHT_SYMTAB_SHNDX sections.

It is allowed to have multiple `SHT_SYMTAB_SHNDX` sections, though
we currently don't implement it.

The current implementation assumes that there is a maximum of one SHT_SYMTAB_SHNDX
section and that it is always linked with .symtab section.

This patch drops this limitations.

Differential revision: https://reviews.llvm.org/D92644