[lld] [ELF] Adjust --compress-sections to support compression level (PR #90567)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> No objections from me. I agree that keeping the default independent of optimisation level is the right thing to do.
> 
> One minor suggestion for the release notes.

Thanks. ISTM adjusting `--compress-sections` seems better (and matches `zlib:1`-style strings I found elsewhere).

It might be useful for `--compress-debug-sections` to get the optional level as well but its use case seems obsoleted by `--compress-sections`...

https://github.com/llvm/llvm-project/pull/90567