[llvm-readobj] Fix "Section" output when emitting relocations in JSON

Prior to this patch, the JSON output would emit an invalid key from the
shared LLVM implementation. This caused llvm-readobj to output invalid
JSON. This patch introduces a small helper function to print the
relocation information differently between the LLVM and JSON formats.

Before this patch:

```
    "Relocations": [Section (2) .rel.text {

      {
        "Relocation": {
          "Offset": 0,
          "Type": {
            "Value": "R_X86_64_NONE",
            "RawValue": 0
          },
          "Symbol": {
            "Value": "rel_0",
            "RawValue": 1
          }
        }
      },
      ...

```

After this patch:

```
   "Relocations": [
      {
        "SectionIdx": 2,
        "Relocs": [
          {
            "Relocation": {
              "Offset": 0,
              "Type": {
                "Name": "R_X86_64_NONE",
                "Value": 0
              },
              "Symbol": {
                "Name": "rel_0",
                "Value": 1
              }
            }
          },
          ...
```

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D137094