8256517: (ref) Reference.clear during reference processing may lose notification

8240696: (ref) Reference.clear may extend the lifetime of the referent

Use private native helper to implement Reference.clear.

Reviewed-by: pliden, rkennke, mchung

src/java.base/share/native/libjava/Reference.c

@@ -49,3 +49,9 @@ Java_java_lang_ref_Reference_refersTo0(JNIEnv *env, jobject ref, jobject o)
 {
     return JVM_ReferenceRefersTo(env, ref, o);
 }
+
+JNIEXPORT void JNICALL
+Java_java_lang_ref_Reference_clear0(JNIEnv *env, jobject ref)
+{
+    JVM_ReferenceClear(env, ref);
+}