[llvm] fix zstd_shared detection on Windows (PR #139945)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo commented:

The changes look reasonable overall, but I would prefer a bit more explanation of what went wrong before, what the previously MSVC specific codepaths do. It could be good to explicitly call it out in the commit message, that this fixes zstd in mingw builds (instead of implicitly via a negation, about windows compilers that aren't msvc).

https://github.com/llvm/llvm-project/pull/139945