Native: fix @ExportObjCClass with the new memory manager

Previous implementation of `@ExportObjCClass` relied on the fact that
initialization of top-level fields was eager, i.e. happened during
Kotlin runtime initialization, when Kotlin code is accesses for the
first time.

With the new MM, initialization of top-level fields became lazy, it
happens when the particular file is accessed for the first time.

This commit fixes `@ExportObjCClass` with the new MM by making the
initialization of particular top-level fields eager.

^KT-53373 Fixed

kotlin-native/backend.native/tests/interop/objc/objcTests.def

@@ -1,6 +1,8 @@
 language = Objective-C
-headers = Foundation/NSArray.h Foundation/NSValue.h Foundation/NSString.h Foundation/NSStream.h Foundation/NSBundle.h
+headers = Foundation/NSArray.h Foundation/NSValue.h Foundation/NSString.h Foundation/NSStream.h Foundation/NSBundle.h \
+    objc/runtime.h
 headerFilter = **/interop/objc/tests/**.h \
     Foundation/NSArray.h Foundation/NSValue.h Foundation/NSString.h Foundation/NSStream.h \
-    objc/objc.h Foundation/NSBundle.h
+    objc/objc.h Foundation/NSBundle.h \
+    objc/runtime.h
 linkerOpts = -lobjctests

kotlin-native/backend.native/tests/interop/objc/tests/kt53373_1.kt

@@ -0,0 +1,5 @@
+import objcTests.*
+import kotlinx.cinterop.*
+
+@ExportObjCClass
+class KT53373 : NSObject()

kotlin-native/backend.native/tests/interop/objc/tests/kt53373_2.kt

@@ -0,0 +1,10 @@
+import kotlinx.cinterop.*
+import kotlin.test.*
+import objcTests.*
+
+@Test
+fun testKT53373() {
+    val kt53373Class = objc_lookUpClass("KT53373")
+    assertNotNull(kt53373Class)
+    assertEquals("KT53373", class_getName(kt53373Class)?.toKString())
+}