toolchain: add new debugging tool using magenta_debug

toolchain/mxdbg/client/attach.c

@@ -0,0 +1,15 @@
+#include "command.h"
+
+static int attach(
+	const struct command *cmd,
+	int argc,
+	const char **argv,
+	void *argp)
+{
+	return 0;
+}
+
+const struct command cmd_attach = {
+	.cmd_name = "attach",
+	.cmd_callback = NULL,
+};