toolchain: add new debugging tool using magenta_debug

toolchain/mxdbg/client/quit.c

@@ -0,0 +1,18 @@
+#include "command.h"
+#include "context.h"
+
+static int quit(
+	const struct command *cmd,
+	struct debug_context *ctx,
+	int argc,
+	const char **argv,
+	void *argp)
+{
+	ctx->ctx_quit = true;
+	return 0;
+}
+
+const struct command cmd_quit = {
+	.cmd_name = "quit",
+	.cmd_callback = quit,
+};