[cmake] fill `cmake_args` when crosscompiling external project with non-clang compiler

[lxbndr]

Summary

This makes it possible to crosscompile runtimes with cl.exe on Windows.

An external project is completely misconfigured otherwise because cmake_args is set only for native builds or builds crosscompiled with clang.

[beanz]

LGTM. Thank you for a nice improvement!

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
           endforeach()
         else()
           list(APPEND cmake_args ${arg})
         endif()
       endforeach()
       foreach(type ${flag_types})
         list(APPEND cmake_args ${${type}_flag})
       endforeach()
+    else()
+      set(cmake_args ${ARG_CMAKE_ARGS})
     endif()
   else()
     set(llvm_config_path "$<TARGET_FILE:llvm-config>")
     set(cmake_args ${ARG_CMAKE_ARGS})
   endif()
 
   if(ARG_TARGET_TRIPLE)
     list(APPEND compiler_args -DCMAKE_C_COMPILER_TARGET=${ARG_TARGET_TRIPLE})