[llvm] [llvm-readobj] Fixes malformed json on JSON printed corefiles (PR #92835)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Totally up to you, but I'd just show the relevant bits.

```JSON
"Mapping": [
     "Start": 4096,
       "End": 8192,
        "Offset": 12288,
         "Filename": "/path/to/a.out"
],
```

```JSON
"Mappings": [
  {
     "Start": 4096,
      "End": 8192,
       "Offset": 12288,
          "Filename": "/path/to/a.out"
  },
 ...
]
```


https://github.com/llvm/llvm-project/pull/92835