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

Change-Id: I9de4a8a00d9fcbad79f6893eff2aa28aafa8e6e3
diff --git a/src/disassembler.cc b/src/disassembler.cc
index ae781dc..5e3f500 100644
--- a/src/disassembler.cc
+++ b/src/disassembler.cc
@@ -27,8 +27,7 @@
   if (instruction_set == kArm || instruction_set == kThumb2) {
     return new arm::DisassemblerArm();
   } else {
-    // TODO: give a better fatal message
-    UNIMPLEMENTED(FATAL);
+    UNIMPLEMENTED(FATAL) << "no disassembler for " << instruction_set;
     return NULL;
   }
 }