tweak zstd behavior in cmake and llvm config for better testing

add LLVM_PREFER_STATIC_ZSTD (default TRUE) cmake config flag
(compression test seems to fail for shared zstd on windows, note that zstd multithread is by default disabled in the static build so it may be a hidden variable)
propagate variable zstd_DIR in LLVMConfig.cmake.in
fix llvm-config CMakeLists.txt behavior for absolute libs windows
get zstd lib name

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D132870