[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