[libclc] [libclc] use default paths with find_program when possible (PR #105969)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Nixpkgs has no intention of moving away from standalone builds.

I encourage you to acquire that intention. IMO libclc should not support the standalone build, and this should be version locked to the exact compiler commit. It's compiler data, not a real library 

https://github.com/llvm/llvm-project/pull/105969