[compiler-rt] Don't redefine LLVM_COMMON_CMAKE_UTILS if it's defined (#66761)

If we are building compiler-rt inside llvm-x.y.z.src directory, we
should not redefine LLVM_COMMON_CMAKE_UTILS.

Author: xry111

compiler-rt/CMakeLists.txt

@@ -5,7 +5,9 @@
 
 cmake_minimum_required(VERSION 3.20.0)
 
-set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
+if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
+  set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
+endif()
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)