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

[Fangrui Song via llvm-commits]

MaskRay wrote:

Sorry. I receive a lot of review requests. When I am not added as a reviewer, I can easily miss a PR that requires my attention... Weekly ping is fine.

> LLVM's build system does the right thing but LLVM_SYSTEM_LIBS ends up containing the shared library. Emit the static library instead when appropriate.

Can you edit the description to show `llvm-config --system-libs` output differences? This would help folks who are less familiar with CMake (like me).

I checked that this patch does change the output of 
`llvm-config --system-libs` 



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