[llvm] [CMake] Add -Wl, -z, pack-relative-relocs if supported (PR #208217)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-test-suite` running on `ppc64le-clang-test-suite` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/95/builds/21527

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

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: LLVM :: Object/macho-invalid.test (112982 of 112992)
PASS: Clang :: OffloadTools/clang-offload-bundler/basic.c (112983 of 112992)
PASS: Clang :: Preprocessor/riscv-target-features.c (112984 of 112992)
PASS: Clang :: Driver/x86-target-features.c (112985 of 112992)
PASS: LLVM :: tools/llvm-objdump/ELF/AMDGPU/subtarget.ll (112986 of 112992)
PASS: Clang :: Driver/linux-ld.c (112987 of 112992)
PASS: LLVM :: Transforms/LoopUnroll/branch-weights-freq/unroll-complete.ll (112988 of 112992)
PASS: Clang :: OpenMP/target_update_codegen.cpp (112989 of 112992)
PASS: Clang :: Driver/range-warnings.c (112990 of 112992)
PASS: LLVM :: CodeGen/RISCV/attributes.ll (112991 of 112992)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=19599.914639

```

</details>

https://github.com/llvm/llvm-project/pull/208217