[llvm-readobj/libObject] - Get rid of `FirstSym` argument. NFCI.

We use `FirstSym` argument in `getExtendedSymbolTableIndex` to calculate
a symbol index:

```
&Sym - &FirstSym
```

Instead, we could pass the symbol index directly.
This is what this patch does, it allows to simplify another llvm-readobj API.

Differential revision: https://reviews.llvm.org/D88016