[llvm-readelf] Implement note parsing for NT_FILE and unknown descriptors

Summary:
This patch implements two note parsers; one for NT_FILE coredumps, e.g.:

```
  CORE                  0x00000080      NT_FILE (mapped files)
    Page size: 4096
                 Start                 End         Page Offset
    0x0000000000001000  0x0000000000002000  0x0000000000003000
        /path/to/a.out
    0x0000000000004000  0x0000000000005000  0x0000000000006000
        /path/to/libc.so
    0x0000000000007000  0x0000000000008000  0x0000000000009000
        [stack]
```

(A more realistic example can be tested locally by creating a crashing program and running `llvm-readelf -n core`)

And also implements a raw hex dump for unknown descriptor data for unhandled descriptor types.

Reviewers: MaskRay, jhenderson, grimar, alexshap

Reviewed By: MaskRay, grimar

Subscribers: emaste, llvm-commits, labath

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65832

llvm-svn: 368698