systemc: Implement the << operator for sc_signal.

Change-Id: I16955e58d96d49ec3bba90b73f5a368a245da438
Reviewed-on: https://gem5-review.googlesource.com/c/12454
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/channel/sc_signal.hh

@@ -173,9 +173,9 @@ class sc_signal : public sc_signal_inout_if<T>,
 
 template <class T, sc_writer_policy WRITER_POLICY>
 inline std::ostream &
-operator << (std::ostream &os, const sc_signal<T, WRITER_POLICY> &)
+operator << (std::ostream &os, const sc_signal<T, WRITER_POLICY> &s)
 {
-    sc_channel_warn_unimpl(__PRETTY_FUNCTION__);
+    os << s.read();
     return os;
 }