gdbstub: add support for extended mode packet

Add support for the '!' extended mode packet. This is required for the
multiprocess extension.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20181207090135.7651-12-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -1302,6 +1302,9 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
     p = line_buf;
     ch = *p++;
     switch(ch) {
+    case '!':
+        put_packet(s, "OK");
+        break;
     case '?':
         /* TODO: Make this return the correct value for user-mode.  */
         snprintf(buf, sizeof(buf), "T%02xthread:%s;", GDB_SIGNAL_TRAP,