espresso: adopt JDK-8365885: Clean up constant pool reflection native code

Move `Target_jdk_internal_reflect_ConstantPool` to standard
substitutions.

Author: gilles-duboscq

espresso/src/com.oracle.truffle.espresso/src/com/oracle/truffle/espresso/libs/libjava/impl/Target_jdk_internal_reflect_ConstantPool.java

@@ -237,4 +237,16 @@ public boolean isValidFor(JavaVersion version) {
             return version.java25OrEarlier();
         }
     }
+
+    final class Java26OrLater implements VersionFilter {
+        public static final Java26OrLater INSTANCE = new Java26OrLater();
+
+        private Java26OrLater() {
+        }
+
+        @Override
+        public boolean isValidFor(JavaVersion version) {
+            return version.java26OrLater();
+        }
+    }
 }