Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/compiler/image_writer.cc b/compiler/image_writer.cc
index 35a3d4b..235aba8 100644
--- a/compiler/image_writer.cc
+++ b/compiler/image_writer.cc
@@ -206,7 +206,7 @@
       break;
     default:
       LOG(FATAL) << "Unreachable.";
-      break;
+      UNREACHABLE();
   }
   object->SetLockWord(LockWord::FromForwardingAddress(offset), false);
   DCHECK(IsImageOffsetAssigned(object));