[llvm] Fix compression header size check in ELF writer (PR #66888)

[Fangrui Song via llvm-commits]

MaskRay wrote:

If someone wants to add a unittest for this case, it would be highly appreciated. But it's fair to say that the author and existing reviewers don't have bandwidth to do this :)

https://github.com/llvm/llvm-project/pull/66888