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

[Fangrui Song via llvm-commits]

MaskRay wrote:

The code checks whether compressing a section is better than not compressing it.

We needed to find a payload where the compressed content is about 12 bytes smaller than the uncompressed content. This is really difficult and would make a test that is too sensitive to different zlib implementations (we have madler, chromium, zlib-ng, etc and many versions to support).

Therefore, I think not having a test in this case is fine.

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