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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Could this be unit tested, then? (I realize the class is currently in the .cpp file, so would have to be moved into a header so it'd be accessible from a unit test)

Likely very brittle to not be dependent on specific zlib versions... Merged as it is better than the wrong status quo..

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