[llvm-readelf] - Implement --addrsig option.

We have `--addrsig` implemented for `llvm-readobj`.
Usually it is convenient to use a single tool for dumping,
so it seems we might want to implement `--addrsig` for `llvm-readelf` too.

I've selected a simple output format which is a bit similar to one,
used for dumping of the symbol table. It looks like:

```
Address-significant symbols section '.llvm_addrsig' contains 2 entries:
   Num: Name
     1: foo
     2: bar
```

Differential revision: https://reviews.llvm.org/D88835