[llvm] [LLVM][CMake] Prefer to find libffi.a if present (PR #78779)

[Petr Hosek via llvm-commits]

petrhosek wrote:

With other dependencies, we use dedicated flags, see for example: https://github.com/llvm/llvm-project/blob/30aa9fb4c1da33892a38f952fbdf6e7e45e5953a/llvm/CMakeLists.txt#L541

Inside the `FindFFI` module, we would ideally expose the static library as a separate target, see for example: https://github.com/llvm/llvm-project/blob/30aa9fb4c1da33892a38f952fbdf6e7e45e5953a/llvm/cmake/modules/Findzstd.cmake#L21-L23

https://github.com/llvm/llvm-project/pull/78779