[lld] [ELF] Retain .rela.dyn for PPC64 PI long-branch thunks (PR #212078)

[Jessica Clarke via llvm-commits]

https://github.com/jrtc27 approved this pull request.

The situation here is a bit sad, ideally we could late-GC the section and reclaim the space in the section headers etc, but this makes sense for where we are today. One comment but otherwise LGTM.

https://github.com/llvm/llvm-project/pull/212078