[clang] [llvm] [MC,llvm-readobj,yaml2obj] Support CREL relocation format (PR #91280)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> I think it would be useful to nominate a source as the canonical reference for the specification which is updated along with any implementation changes. I think [discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600/3](https://discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600/3) is a good starting point.
> 
> Possible examples:
> 
> * A patch to the LLVM documentation.
> * A blog-post that is kept up to date with the implementation.
> * A discourse post that is updated.

Thanks. I frequently update my blog posts and https://maskray.me/blog/2024-03-09-a-compact-relocation-format-for-elf#crel-relocation-format will be kept up to date.

However, I feel not so good to name my personal website. Then, https://discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600/3 should serve the purpose well. discourse.llvm.org posts become non-editable after a period of time. I'll need to make further applies if changes are made.

> Link: [discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600](https://discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600)

I can edit the link to append `/3` to refer to the specification.

https://github.com/llvm/llvm-project/pull/91280