Provide CName annotation in common stdlib as optional

kotlin-native/runtime/src/main/kotlin/kotlin/native/Annotations.kt

@@ -50,5 +50,5 @@ public typealias SharedImmutable = kotlin.native.concurrent.SharedImmutable
  */
 @Target(AnnotationTarget.FUNCTION)
 @Retention(AnnotationRetention.BINARY)
-public annotation class CName(val externName: String = "", val shortName: String = "")
+public actual annotation class CName(actual val externName: String = "", actual val shortName: String = "")
 

libraries/stdlib/src/kotlin/annotations/NativeAnnotations.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin.native
+
+/**
+ * Makes top level function available from C/C++ code with the given name.
+ *
+ * [externName] controls the name of top level function, [shortName] controls the short name.
+ * If [externName] is empty, no top level declaration is being created.
+ */
+@SinceKotlin("1.5")
+@Target(AnnotationTarget.FUNCTION)
+@Retention(AnnotationRetention.BINARY)
+@OptionalExpectation
+public expect annotation class CName(val externName: String = "", val shortName: String = "")