gdbstub: Add a missing case of signal number translation in gdbstub

While using qemu with gdb "target remote" to debug an application that uses
fork and exec, the qemu process receives SIGSTOP every time the forked process
terminates (sending SIGCHLD).

This is caused by a missing call to gdb_signal_to_target in gdbstub.c, which
is fixed by this patch:

Signed-off-by: Martin Simmons <martin@lispworks.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

gdbstub.c

@@ -823,7 +823,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
                 action = *p++;
                 signal = 0;
                 if (action == 'C' || action == 'S') {
-                    signal = strtoul(p, (char **)&p, 16);
+                    signal = gdb_signal_to_target(strtoul(p, (char **)&p, 16));
+                    if (signal == -1) {
+                        signal = 0;
+                    }
                 } else if (action != 'c' && action != 's') {
                     res = 0;
                     break;