[lld] [ELF] Support relocatable files using CREL (PR #98115)

[Peter Smith via llvm-commits]

smithp35 wrote:

I've taken a look at the tests. I'm now down to a few last questions, triggered by the no mixing of SHT_REL and CREL in emit-relocs and -r links.

I was trying to work out why it would be complex. As I understand it:
* CREL in relocatable objects uses explicit addends even when it is usual to use SHT_REL. (From Addend bit: in https://maskray.me/blog/2024-03-09-a-compact-relocation-format-for-elf)
* To convert from SHT_REL to CREL would require extracting the implicit addends 

https://github.com/llvm/llvm-project/pull/98115