Proper getStackAccessControlContext implementation

Fixed imports and warnings in SecuritySubstitutions.
Implemented PrivilegedStack using FastThreadLocal.
Added missing getProtectionDomain method.
Disallowed NO_CONTEXT_SINGLETON in executePrivileged.
Added allowlist for known-good JDK contexts.

Author: lazar-mitrovic

sdk/src/org.graalvm.nativeimage/src/org/graalvm/nativeimage/impl/ProcessPropertiesSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * The Universal Permissive License (UPL), Version 1.0
@@ -45,7 +45,9 @@
 import org.graalvm.nativeimage.c.function.CEntryPointLiteral;
 
 public interface ProcessPropertiesSupport {
-    String getExecutableName();
+    default String getExecutableName() {
+        return "java";
+    }
 
     long getProcessID();