[llvm-readobj] Implement GNU-style output for dynamic table

[llvm-readobj] Implement GNU-style output for dynamic table

GNU readelf tool prints slightly different dynamic table "header" and
surrounds dynamic tag names by brackets. This patch implements the same
formatting for GNU-style output of the `llvm-readobj`.

LLVM
```
DynamicSection [ (13 entries)
  Tag        Type                 Name/Value
  0x00000006 SYMTAB               0x168
  ...
]
```

GNU
```
Dynamic section at offset 0x1d0 contains 13 entries:
  Tag        Type                 Name/Value
  0x00000006 (SYMTAB)             0x168
  ...
```

Differential Revision: https://reviews.llvm.org/D62256

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361633 91177308-0d34-0410-b5e6-96231b3b80d8