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

[Fangrui Song via llvm-commits]

MaskRay wrote:

+1 for a new CMake variable. Many distributions prefer dynamic linking. Linking in `libffi.a` could cause a conflict (usually benign) if a program links against both `libLLVM` and `libffi.so`.

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