Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 9c04133..da82c76 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -1971,6 +1971,7 @@
   exception_handler.UpdateInstrumentationStack();
   exception_handler.DoLongJump();
   LOG(FATAL) << "UNREACHABLE";
+  UNREACHABLE();
 }
 
 Context* Thread::GetLongJumpContext() {