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

Summary:
This patch is an attempt to make the `find_package(FFI)` support in LLVM
prefer to provide the static library version if present. This is
currently
an optional library for building `libffi`, and its presence implies that
it should likely be used. This patch is an attempt to fix some problems
observed with testing programs linked against `libffi` on many different
systems that could have conflicting paths. Linking it statically
prevents this.

This patch adds the `ffi_static` target for this library.