[cmake] Fix cmake warning in standalone compiler-rt builds.

```
cd compiler-rt/build
cmake -G Ninja ../ -DCOMPILER_RT_STANDALONE_BUILD=ON
-DLLVM_CONFIG_PATH=<...>llvm-project/build/bin/llvm-config
-DCOMPILER_RT_INCLUDE_TESTS=ON
```

```
-- check-shadowcallstack does nothing.
Traceback (most recent call last):
  File "<string>", line 22, in <module>
  IndexError: list index out of range
  -- Configuring done
  -- Generating done
```

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D88957

llvm/cmake/modules/AddLLVM.cmake

@@ -1494,6 +1494,7 @@ def relpath(p):\n
     if os.path.splitdrive(p)[0] != os.path.splitdrive(base)[0]: return p\n
     if haslink(p) or haslink(base): return p\n
     return os.path.relpath(p, base)\n
+if len(sys.argv) < 3: sys.exit(0)\n
 sys.stdout.write(';'.join(relpath(p) for p in sys.argv[2].split(';')))"
     ${basedir}
     ${pathlist_escaped}