fmt,cxxrtl: add option to group digits in numbers.

The option is serialized to RTLIL as `_` (to match Python's option with
the same symbol), and sets the `group` flag. This flag inserts an `_`
symbol between each group of 3 digits (for decimal) or four digits (for
binary, hex, and octal).

kernel/fmt.h

@@ -85,6 +85,7 @@ struct FmtPart {
 	} sign = MINUS;
 	bool hex_upper = false;
 	bool show_base = false;
+	bool group = false;
 
 	// VLOG_TIME type
 	bool realtime = false;