Add Z3 to system libraries list if enabled

Without this trying to link static LLVM libraries (built with Z3 enabled) fails because `llvm-config` doesn't print `-lz3`.
We are already using this patch at MSYS2: https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-clang/0013-Add-Z3-to-system-libraries-list-if-enabled.patch

Reviewed By: mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D85195