[LIBCLC] Correctly set up llvm-spirv and libclc-remangler

[jchlanda]

Customs dirs path was asserting on the existence of llvm-spirv and libclc-remangler, however those tools have not been set up.

Also only search for llvm-spirv if it has not been set in the custom tools path.

[jchlanda]

cc: @frasercrmck

[jchlanda]

@frasercrmck are you ok with this going in?

[jchlanda]

@intel/llvm-gatekeepers this is ready to go. Thank you.