Give CPUFeatures::kNone a stable value.

This makes the simulation pseudo-instruction mechanism a bit more stable across
VIXL versions.

We still don't guarantee that simulation pseusdo-instructions generated with the
MacroAssembler will work with a Simulator from a different version of VIXL, but
this patch has no real cost for VIXL, and might make things easier for someone.

Change-Id: Idd92328801a88f54a8d30db86304746a22ccb1df
diff --git a/src/cpu-features.h b/src/cpu-features.h
index 853421b..f94b955 100644
--- a/src/cpu-features.h
+++ b/src/cpu-features.h
@@ -167,11 +167,11 @@
     // Refer to VIXL_CPU_FEATURE_LIST (above) for the list of feature names that
     // this class supports.
 
+    kNone = -1,
 #define VIXL_DECLARE_FEATURE(SYMBOL, NAME, CPUINFO) SYMBOL,
     VIXL_CPU_FEATURE_LIST(VIXL_DECLARE_FEATURE)
 #undef VIXL_DECLARE_FEATURE
-    kNone,
-    kNumberOfFeatures = kNone
+    kNumberOfFeatures
   };
   // clang-format on