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

[pgousseau]

Summary

LLVM_LIT_CONFIG_FILES is not defined in standalone compiler-rt builds, causing make_paths_relative to fail.

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

llvm/cmake/modules/AddLLVM.cmake

 def haslink(p):\n
     if not p or p == os.path.dirname(p): return False\n
     return os.path.islink(p) or haslink(os.path.dirname(p))\n
 def relpath(p):\n
     if not p: return ''\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}
     ${pathlist_escaped}
     OUTPUT_VARIABLE pathlist_relative)
   set(${out_pathlist} "${pathlist_relative}" PARENT_SCOPE)
 endfunction()
 
 # Converts a file that's relative to the current python file to an absolute