[clang] [clang] Add information about lld presence in RISCVToolchain. (PR #68904)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @MaskRay, could you please look into this?

Sorry for my late reply. I am still adapting to the github pr workflow.

Related to the linker question you have, I am puzzled by RISCVToolChain's choices: https://github.com/llvm/llvm-project/pull/71803#issuecomment-1804648888

https://github.com/llvm/llvm-project/pull/68904