gdbstub: don't report auxv feature unless on Linux

The later handler if conditionally compiled only for Linux but we
forgot to ensure we don't advertise it lest we confuse our BSD
brethren.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Fixes: 51c623b ("gdbstub: add support to Xfer:auxv:read: packet")
Reported-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Tested-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230403134920.2132362-4-alex.bennee@linaro.org>

gdbstub/gdbstub.c

@@ -1468,7 +1468,7 @@ static void handle_query_supported(GArray *params, void *user_ctx)
             ";ReverseStep+;ReverseContinue+");
     }
 
-#ifdef CONFIG_USER_ONLY
+#if defined(CONFIG_USER_ONLY) && defined(CONFIG_LINUX)
     if (gdbserver_state.c_cpu->opaque) {
         g_string_append(gdbserver_state.str_buf, ";qXfer:auxv:read+");
     }