Add the missing operator<< for InstructionSet and use it in the disassembler.

Change-Id: I9de4a8a00d9fcbad79f6893eff2aa28aafa8e6e3
diff --git a/src/constants.h b/src/constants.h
index bb17c2d..832d6f7 100644
--- a/src/constants.h
+++ b/src/constants.h
@@ -29,6 +29,8 @@
   kMips
 };
 
+std::ostream& operator<<(std::ostream& os, const InstructionSet& rhs);
+
 enum InvokeType {
   kStatic, kDirect, kVirtual, kSuper, kInterface,
   kMaxInvokeType = kInterface