[obj2yaml] - Stop dumping an empty sh_info field for SHT_RELA/SHT_REL sections.

`.rela.dyn` is a dynamic relocation section that normally has
no value in `sh_info` field.

The existent `elf-reladyn-section-shinfo.yaml` which tests this piece has issues:

1) It does not check the case when we have more than one `SHT_REL[A]`
   section with `sh_info == 0` in the object. Because of this it did not catch the issue.
   Currently we print an excessive "Info" field:

```
  - Name:            .rela.dyn
    Type:            SHT_RELA
    EntSize:         0x0000000000000018
  - Name:            .rel.dyn
    Type:            SHT_REL
    EntSize:         0x0000000000000010
    Info:            ' [1]'
```

2) It seems can be more generic. I've added a `rel-rela-section.yaml` instead.

Differential revision: https://reviews.llvm.org/D76281