[llvm] Add function merger to be run during LTO link with gold plugin (PR #121343)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vla` running on `linaro-g3-01` while building `llvm` at step 1 "Checkout lnt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/17/builds/5128

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

```
Step 1 (Checkout lnt) failure: update (failure)
git version 2.34.1
fatal: unable to access 'https://github.com/llvm/llvm-lnt.git/': gnutls_handshake() failed: The TLS connection was non-properly terminated.
fatal: unable to access 'https://github.com/llvm/llvm-lnt.git/': gnutls_handshake() failed: The TLS connection was non-properly terminated.

```

</details>

https://github.com/llvm/llvm-project/pull/121343