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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> 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.

Agreed. This provides another data point (we are short of such tests) so that this may hopefully be refactored in the future.

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