Rustc-dev-guide: Introduce something more about Codegen and LLVM

[crlf0710]

Hello i'm trying to make a little progress on the raw_dylib rfc implementation. It seems this RFC needs to deal with the codegen process, and i'm seeking for more introductory materials on this affair. It will be great if this can be introduced more in rustc-guide. Thanks!

[mark-i-m]

@crlf0710 Yes, this is one of the biggest gaps in the guide that I've been wanting to fill for a while.

Would you be willing to write down what you learned?

@spastorino perhaps we can schedule a zulip discussion with nagisa

[crlf0710]

Sure, I'm happy to help with the guide as much as i can.

[bjorn3]

I wrote some advice from my experience writing a codegen backend at https://github.com/bjorn3/rustc_codegen_cranelift/issues/647. Maybe include some of it in the guide?