Add a separate aggressive GC mode

kotlin-native/runtime/src/main/cpp/Runtime.cpp

@@ -31,13 +31,18 @@ struct InitNode {
   InitNode* next;
 };
 
-// This global is overriden by the compiler.
+// These globals are overriden by the compiler.
 RUNTIME_WEAK DestroyRuntimeMode Kotlin_destroyRuntimeMode = DESTROY_RUNTIME_ON_SHUTDOWN;
+RUNTIME_WEAK KInt Kotlin_gcAggressive = 0;
 
 DestroyRuntimeMode Kotlin_getDestroyRuntimeMode() {
     return Kotlin_destroyRuntimeMode;
 }
 
+bool Kotlin_getGcAggressive() {
+    return Kotlin_gcAggressive != 0;
+}
+
 namespace {
 
 InitNode* initHeadNode = nullptr;