Update example for Extension Event in jvmti specs

Summary

The JVMTI specification describes the "Extension Event" and gives an example, but the example doesn't match the specification. The example must be changed to match the specification. This is a clear copy of the same CSR for openjdk16.

Problem

In the JVMTI specification, in the description of "Extension Event", the specification for the extension event callback function is given as:

typedef void (JNICALL *jvmtiExtensionEvent)
     (jvmtiEnv* jvmti_env,
      ...);

But the example for a handler that takes a jint parameter is given as:

void JNICALL myHandler(jvmtiEnv* jvmti_env, jint myInt, ...)

Here a variadic function argument (myInt) is declared as a constant argument. On some platforms constant and variadic arguments are passed differently, and following the example will result in error on such platforms.

Solution

Change the example function to match the specification of jvmtiExtensionEvent and only take a varargs parameter. Change IsClassUnloadingEnabled function in hotspot to match updated spec.

Specification

src/hotspot/share/prims/jvmti.xml

           there is a <code>jint</code> parameter, the event handler should be
           declared:
 <example>
-    void JNICALL myHandler(jvmtiEnv* jvmti_env, jint myInt, ...)
+    void JNICALL myHandler(jvmtiEnv* jvmti_env, ...)
 </example>
           Note the terminal "<code>...</code>" which indicates varargs.
+          The <code>jint</code> argument inside <code>myHandler</code> needs to be extracted using
+          the <code>va_*</code> syntax of the C programming language.
         </description>
         <parameters>
           <param id="jvmti_env">

and additionally in jvmtiExtensions.cpp

-static jvmtiError JNICALL IsClassUnloadingEnabled(const jvmtiEnv* env, jboolean* enabled, ...) {
+static jvmtiError JNICALL IsClassUnloadingEnabled(const jvmtiEnv* env, ...) {