sim/rl78: Add ATTRIBUTE_PRINTF

Clang generates a warning if the format string of a printf-like function is
not a literal ("-Wformat-nonliteral").  On the default configuration, it
causes a build failure (unless "--disable-werror" is specified).

To avoid warnings on the printf-like wrapper, it requires proper
__attribute__((format)) and we have ATTRIBUTE_PRINTF macro for this reason.

This commit adds ATTRIBUTE_PRINTF to the printf-like functions.

sim/rl78/trace.c

@@ -92,7 +92,7 @@ compare_symbols (const void *ap, const void *bp)
 
 static char opbuf[1000];
 
-static int
+static int ATTRIBUTE_PRINTF (2, 3)
 op_printf (char *buf, char *fmt, ...)
 {
   int ret;
@@ -104,7 +104,7 @@ op_printf (char *buf, char *fmt, ...)
   return ret;
 }
 
-static int
+static int ATTRIBUTE_PRINTF (3, 4)
 op_styled_printf (char *buf, enum disassembler_style style, char *fmt, ...)
 {
   int ret;