[libclc] [llvm] [libclc] Replace llvm-dis with llvm-nm in check-external-funcs.test (PR #187190)

[Wenju He via cfe-commits]

wenju-he wrote:

> > It doesn't require a script and also works on windows.
> 
> You could fully implement this in cmake without depending on bash

done in the cmake

https://github.com/llvm/llvm-project/pull/187190