Add Android System#arraycopy fast specializations.

Re-added System#arraycopy specializations that are
really fast for small arrays. They were introduced
originally in b/7103825.

Change-Id: Ie8c995852810ad555fe73743d4d40ec45d8d6bbe
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index f9f50b4..3f27e44 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -997,6 +997,7 @@
   REGISTER(register_java_lang_DexCache);
   REGISTER(register_java_lang_Object);
   REGISTER(register_java_lang_Thread);
+  REGISTER(register_java_lang_System);
   REGISTER(register_java_lang_VMClassLoader);
   REGISTER(register_java_lang_ref_FinalizerReference);
   REGISTER(register_java_lang_ref_Reference);