Use the UNIMPLEMENTED macro everywhere.

As suggested in review.

Change-Id: Ic14c91a622b545b4800ef021dfb501fb09cbda9d
diff --git a/src/assembler_arm.cc b/src/assembler_arm.cc
index 70dd20c..b75263a 100644
--- a/src/assembler_arm.cc
+++ b/src/assembler_arm.cc
@@ -1508,7 +1508,7 @@
       mov(dest.AsCoreRegister(), ShifterOperand(src.AsCoreRegister()));
     } else {
       // TODO: VFP
-      LOG(FATAL) << "Unimplemented";
+      UNIMPLEMENTED(FATAL) << ": VFP";
     }
   }
 }