Use ompstub by default on Windows on ARM

OpenMP is not supported on Windows on ARM yet.
Therefore this PR modified Cmakelist to use ompstub
on Windows on ARM for linking by default.

Author: kaadam

runtime/flangrti/CMakeLists.txt

@@ -71,21 +71,26 @@ add_flang_library(flangrti_shared
   )
 
 # Resolve symbols against libm
-target_link_libraries(flangrti_shared m)
+target_link_libraries(flangrti_shared PRIVATE m)
 
 # Resolve symbols against libpthread
 find_package(Threads REQUIRED)
 if (CMAKE_THREAD_LIBS_INIT)
-  target_link_libraries(flangrti_shared "${CMAKE_THREAD_LIBS_INIT}")
+  target_link_libraries(flangrti_shared PRIVATE "${CMAKE_THREAD_LIBS_INIT}")
 endif()
 
 # Import OpenMP
 if (NOT DEFINED LIBOMP_EXPORT_DIR)
-  find_library( 
-    FLANG_LIBOMP
-    libomp.so
-    HINTS ${CMAKE_BINARY_DIR}/lib)
-  target_link_libraries(flangrti_shared ${FLANG_LIBOMP})
+  # OpenMP is not supported on Windows on ARM yet, use ompstub for linking by default
+  if(NOT (MSVC AND ${TARGET_ARCHITECTURE} STREQUAL "aarch64"))
+    find_library(
+      FLANG_LIBOMP
+      NAMES omp libomp
+      HINTS ${CMAKE_BINARY_DIR}/lib)
+    target_link_libraries(flangrti_shared PRIVATE ${FLANG_LIBOMP})
+  else()
+    target_link_libraries(flangrti_shared PRIVATE ompstub_shared)
+  endif()
 endif()
 
 find_library(