Add extmodule support to firrtl backend

The current firrtl backend emits blackboxes as standard modules
with an empty body, but this causes the firrtl compiler to
optimize out entire circuits due to the absence of any drivers.

Yosys already tags blackboxes with a (*blackbox*) attribute, so this
commit just propagates this change to firrtl's syntax for blackboxes.

backends/firrtl/firrtl.cc

@@ -392,7 +392,37 @@ struct FirrtlWorker
 		return result;
 	}
 
-	void run()
+	void emit_extmodule()
+	{
+		std::string moduleFileinfo = getFileinfo(module);
+		f << stringf("  extmodule %s: %s\n", make_id(module->name), moduleFileinfo.c_str());
+		vector<string> port_decls;
+
+		for (auto wire : module->wires())
+		{
+			const auto wireName = make_id(wire->name);
+			std::string wireFileinfo = getFileinfo(wire);
+
+			// Maybe not needed?
+			if (wire->port_id)
+			{
+				if (wire->port_input && wire->port_output)
+				{
+					log_error("Module port %s.%s is inout!\n", log_id(module), log_id(wire));
+				}
+				port_decls.push_back(stringf("    %s %s: UInt<%d> %s\n", wire->port_input ? "input" : "output",
+						wireName, wire->width, wireFileinfo.c_str()));
+			}
+		}
+
+		for (auto str : port_decls) {
+			f << str;
+		}
+
+		f << stringf("\n");
+	}
+
+	void emit_module()
 	{
 		std::string moduleFileinfo = getFileinfo(module);
 		f << stringf("  module %s: %s\n", make_id(module->name), moduleFileinfo.c_str());
@@ -1076,6 +1106,22 @@ struct FirrtlWorker
 
 		for (auto str : wire_exprs)
 			f << str;
+
+		f << stringf("\n");
+	}
+
+	void run()
+	{
+		// Blackboxes should be emitted as `extmodule`s in firrtl. Only ports are
+		// emitted in such a case.
+		if (module->get_blackbox_attribute())
+		{
+			emit_extmodule();
+		}
+		else
+		{
+			emit_module();
+		}
 	}
 };