[Native] Implement 'kotlinx.cinterop.UnsafeNumber' interface

^KT-48459

kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Annotations.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 kotlinx.cinterop
+
+/**
+ * Marker for typealias that will represent numbers of different bit width on at least two platforms.
+ *
+ * @param actualPlatformTypes: Contains the underlying platform types represented as `{konanTarget}: {type fqn}`
+ * e.g. ["linux_x64: kotlin.Int", "linux_arm64: kotlin.Long"]
+ */
+@Suppress("unused") // Is emitted by the Commonizer
+@Target(AnnotationTarget.TYPEALIAS)
+@Retention(AnnotationRetention.BINARY)
+@RequiresOptIn(level = RequiresOptIn.Level.WARNING)
+annotation class UnsafeNumber(val actualPlatformTypes: Array<String>)