[libclc] [libclc] Update build instructions in readme (PR #111369)

[David Spickett via cfe-commits]

DavidSpickett wrote:

Also do you know if libclc has any tests worth running in CI? It doesn't have a check-libclc so I'm not sure if that's because there's not much to run, or running the tests requires a GPU or something else.

Context is I've been looking at the pipeline we use for PRs and it doesn't know what to do with libclc so it uses check-all, ideally it would have a specific target.

https://github.com/llvm/llvm-project/pull/111369