[llvm] [llvm] [cmake] Use pkg-config to obtain libffi search hints (PR #144221)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-release` running on `linaro-flang-aarch64-release` while building `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/172/builds/13188

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-flang) failure: test (failure)
******************** TEST 'Flang :: Semantics/modfile76.F90' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/flang -fc1 -fsyntax-only -fhermetic-module-files -DSTEP=1 /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90 # RUN: at line 1
+ /home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/flang -fc1 -fsyntax-only -fhermetic-module-files -DSTEP=1 /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90
/home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/flang -fc1 -fsyntax-only /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90 # RUN: at line 2
+ /home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/flang -fc1 -fsyntax-only /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90
error: Semantic errors in /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90
/home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Semantics/modfile76.F90:22:12: error: No explicit type declared for 'x'
    print *, x
             ^

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/144221