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

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Thanks! 

I have pushed for RELR adoption in the past. This configure time check looks good to me. There is indeed no good alternative detection.

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