Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/interpreter/interpreter.cc b/runtime/interpreter/interpreter.cc
index dfb03cd..9de12f2 100644
--- a/runtime/interpreter/interpreter.cc
+++ b/runtime/interpreter/interpreter.cc
@@ -325,7 +325,7 @@
 JValue ExecuteGotoImpl(Thread* self, MethodHelper& mh, const DexFile::CodeItem* code_item,
                        ShadowFrame& shadow_frame, JValue result_register) {
   LOG(FATAL) << "UNREACHABLE";
-  exit(0);
+  UNREACHABLE();
 }
 // Explicit definitions of ExecuteGotoImpl.
 template<> SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)