Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/entrypoints/quick/quick_alloc_entrypoints.cc b/runtime/entrypoints/quick/quick_alloc_entrypoints.cc
index a2869ec..7dbfdd5 100644
--- a/runtime/entrypoints/quick/quick_alloc_entrypoints.cc
+++ b/runtime/entrypoints/quick/quick_alloc_entrypoints.cc
@@ -248,7 +248,8 @@
     }
 #endif
     default: {
-      LOG(FATAL) << "Unimplemented";
+      UNIMPLEMENTED(FATAL);
+      UNREACHABLE();
     }
   }
 }