Native: rewrite Kotlin_Interop_getObjCClass in Objective-C

This allows calling it in "Native" thread state.

kotlin-native/runtime/src/main/cpp/ObjCInterop.mm

@@ -173,7 +173,11 @@ void releaseAsAssociatedObjectImp(id self, SEL _cmd, ReleaseMode mode) {
 
 extern "C" {
 
-Class Kotlin_Interop_getObjCClass(const char* name);
+Class Kotlin_Interop_getObjCClass(const char* name) {
+    Class result = objc_lookUpClass(name);
+    RuntimeCheck(result != nil, "Objective-C class '%s' not found. Ensure that the containing framework or library was linked.", name);
+    return result;
+}
 
 const TypeInfo* GetObjCKotlinTypeInfo(ObjHeader* obj) RUNTIME_NOTHROW;
 
@@ -391,10 +395,6 @@ void Kotlin_objc_release(id ptr) {
   objc_release(ptr);
 }
 
-Class Kotlin_objc_lookUpClass(const char* name) {
-  return objc_lookUpClass(name);
-}
-
 } // extern "C"
 
 #else  // KONAN_OBJC_INTEROP
@@ -426,11 +426,6 @@ void Kotlin_objc_release(void* ptr) {
   RuntimeAssert(false, "Objective-C interop is disabled");
 }
 
-void* Kotlin_objc_lookUpClass(const char* name) {
-  RuntimeAssert(false, "Objective-C interop is disabled");
-  return nullptr;
-}
-
 } // extern "C"
 
 #endif // KONAN_OBJC_INTEROP