linux-user: Don't omit comma for strace of rt_sigaction()

Pass the 'last' parameter of print_signal() through to
print_raw_param(); this fixes a problem where we weren't printing
the comma separator for strace of rt_sigaction() when the signal
was an unnamed (ie realtime) one:
  6856 rt_sigaction(230xf6fff870,0xf6fff8fc) = 0
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

linux-user/strace.c

@@ -143,7 +143,7 @@ print_signal(abi_ulong arg, int last)
     case TARGET_SIGTTOU: signal_name = "SIGTTOU"; break;
     }
     if (signal_name == NULL) {
-        print_raw_param("%ld", arg, 1);
+        print_raw_param("%ld", arg, last);
         return;
     }
     gemu_log("%s%s", signal_name, get_comma(last));