Fix linux build

Author: aschwaighofer

stdlib/public/stubs/CMakeLists.txt

@@ -42,5 +42,7 @@ add_swift_library(swiftStdlibStubs OBJECT_LIBRARY TARGET_LIBRARY
   LINK_FLAGS ${SWIFT_RUNTIME_CORE_LINK_FLAGS}
   INSTALL_IN_COMPONENT stdlib)
 
-set_property(SOURCE SwiftNativeNSXXXBaseARC.m APPEND_STRING PROPERTY COMPILE_FLAGS
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+  set_property(SOURCE SwiftNativeNSXXXBaseARC.m APPEND_STRING PROPERTY COMPILE_FLAGS
     "-fobjc-arc")
+endif()