[llvm] [llvm-config] Make llvm-config --system-libs obey LLVM_USE_STATIC_ZSTD. (PR #93754)

[Michał Górny via llvm-commits]

https://github.com/mgorny approved this pull request.

I don't see anything wrong with it, though I really hate that we have to do things like that.

https://github.com/llvm/llvm-project/pull/93754