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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Thankyou! So I think in CI we can not add any extra testing targets if libclc is included in the build.

If you have some in future I'm happy to help enable them.

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