[test] Avoid building unnecessary targets when cross-compiling non-Darwin platforms

Author: kateinoigakukun

stdlib/private/CMakeLists.txt

@@ -23,7 +23,8 @@ endif()
 if(SWIFT_BUILD_SDK_OVERLAY)
   add_subdirectory(OSLog)
 
-  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+  list_intersect("${SWIFT_APPLE_PLATFORMS}" "${SWIFT_SDKS}" building_darwin_sdks)
+  if(building_darwin_sdks)
     add_subdirectory(StdlibUnittestFoundationExtras)
   endif()
   # Currently SwiftReflectionTest cannot be built on Windows, due to