[llvm] [llvm-objdump] Implement decoding auxiliary header for xcoff with llvm-objdump --private-headers (PR #105682)

[James Henderson via llvm-commits]

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+## Test that `the llvm-objdump --private-headers` prints out the auxiliary header of XCOFF object file.
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jh7370 wrote:

```suggestion
## Test that `llvm-objdump --private-headers` prints out the auxiliary header of an XCOFF object file.
```

https://github.com/llvm/llvm-project/pull/105682