[K/N] Add kotlinx.cinterop.ExperimentalForeignApi opt-ins

Add more opt-ins in Samples, tests and Benchmarks.
This is a follow-up to KT-MR-9788 

Merge-request: KT-MR-9997
Merged-by: Pavel Punegov <Pavel.Punegov@jetbrains.com>

kotlin-native/backend.native/tests/link/purge1/lib.kt

@@ -2,6 +2,7 @@
  * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
+@file:OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
 
 import kotlinx.cinterop.convert
 import platform.posix.*

kotlin-native/backend.native/tests/samples/build.gradle.kts

@@ -21,7 +21,10 @@ allprojects {
     }
 
     tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinNativeCompile>().configureEach {
-        compilerOptions.freeCompilerArgs.add("-XXLanguage:+ImplicitSignedToUnsignedIntegerConversion")
+        compilerOptions.freeCompilerArgs.addAll(
+                "-XXLanguage:+ImplicitSignedToUnsignedIntegerConversion",
+                "-opt-in=kotlinx.cinterop.ExperimentalForeignApi"
+        )
     }
 }