[K/N] Internalize getNativeNullPtr() functions

As a part of efforts to stabilize Native stdlib.

kotlin-native/Interop/Runtime/src/native/kotlin/kotlinx/cinterop/NativeTypes.kt

@@ -16,7 +16,6 @@
 
 package kotlinx.cinterop
 
-import kotlin.native.internal.getNativeNullPtr
 import kotlin.native.internal.reinterpret
 import kotlin.native.internal.Intrinsic
 import kotlin.native.internal.VolatileLambda
@@ -30,7 +29,7 @@ internal typealias NonNullNativePtr = kotlin.native.internal.NonNullNativePtr
 internal inline fun NativePtr.toNonNull() = this.reinterpret<NativePtr, NonNullNativePtr>()
 
 inline val nativeNullPtr: NativePtr
-    get() = getNativeNullPtr()
+    get() = NativePtr.NULL
 
 @Deprecated("Use sizeOf<T>() or alignOf<T>() instead.")
 @Suppress("DEPRECATION")

kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/NativePtr.kt

@@ -8,7 +8,8 @@
 package kotlin.native.internal
 
 @TypedIntrinsic(IntrinsicType.INTEROP_GET_NATIVE_NULL_PTR)
-external fun getNativeNullPtr(): NativePtr
+@PublishedApi
+internal external fun getNativeNullPtr(): NativePtr
 
 class NativePtr @PublishedApi internal constructor(private val value: NonNullNativePtr?) {
     companion object {