Clean up some A64 specific code in common code that was introduced by A64 merge

src/a64/simulator-a64.h

Index: src/a64/simulator-a64.h
diff --git a/src/a64/simulator-a64.h b/src/a64/simulator-a64.h
index 6279677ca8565e46f32d4ce8caafed06a3d6b1b7..b1f0b826d1df944511432284aa86d17ae13593bb 100644
--- a/src/a64/simulator-a64.h
+++ b/src/a64/simulator-a64.h
@@ -254,7 +254,9 @@ class Simulator : public DecoderVisitor {
     }
 
     explicit CallArgument(float argument) {
-      TODO_UNIMPLEMENTED("CallArgument(float) is untested.");
+      // TODO(all): CallArgument(float) is untested, remove this check once
+      //            tested.
+      UNIMPLEMENTED();
       // Make the D register a NaN to try to trap errors if the callee expects a
       // double. If it expects a float, the callee should ignore the top word.
       ASSERT(sizeof(kFP64SignallingNaN) == sizeof(bits_));