[llvm] [yaml2obj][XOFF] Update yaml2obj for XCOFF to create valid XCOFF files in more cases. (PR #77620)

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stephenpeckham wrote:

> > Also you need to ensure that all your optional fields are tested (showing default behaviour when not specified).
> 
> Have you addressed this comment?
Adding tests for every optional field is beyond the scope of this PR. 


https://github.com/llvm/llvm-project/pull/77620