diff --git a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
--- a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -51,6 +51,13 @@
       string(REGEX REPLACE "^-DCMAKE_SYSTEM_NAME=(.*)$" "\\1" _cmake_system_name "${arg}")
     endif()
   endforeach()
+  if(NOT _cmake_system_name)
+    foreach(arg ${${nameCanon}_CMAKE_ARGS})
+      if(arg MATCHES "^-DCMAKE_SYSTEM_NAME=")
+        string(REGEX REPLACE "^-DCMAKE_SYSTEM_NAME=(.*)$" "\\1" _cmake_system_name "${arg}")
+      endif()
+    endforeach()
+  endif()
 
   if(NOT ARG_TOOLCHAIN_TOOLS)
     set(ARG_TOOLCHAIN_TOOLS clang lld llvm-ar llvm-lipo llvm-ranlib llvm-nm llvm-objdump)