[openmp] [llvm] [LLVM][CMake] Add ffi_static target for the FFI static library (PR #78779)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> I am not familiar with CMake, but should there be a `LLVM_USE_STATIC_FFI` to select `libffi.a` when both `libffi.so` and `libffi.a` are present?

The user can decide which one to use based off of whether or not `FFI_STATIC_LIBRARIES` was set, as in the changed code in `libomptarget`.

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