backend.native: support external functions

also implement changing external function symbol name with the annotation

runtime/src/main/kotlin/kotlin_native/annotations.kt

@@ -0,0 +1,11 @@
+package kotlin_native
+
+/**
+ * Forces the compiler to use specified symbol name for the target `external` function.
+ *
+ * TODO: changing symbol name breaks the binary compatibility,
+ * so it should probably be allowed on `internal` and `private` functions only.
+ */
+@Target(AnnotationTarget.FUNCTION)
+@Retention(AnnotationRetention.BINARY)
+annotation class SymbolName(val name: String)