[lld] [LLD][ELF] Reduce memory and file size of overlay thunk tests (PR #222565)

[Peter Smith via llvm-commits]

smithp35 wrote:

As an aside, while updating the tests I ran into some likely issues with OVERLAY and PHDRS, which was my first attempt to fix. Will open issues to record what I found.

https://github.com/llvm/llvm-project/pull/222565