selftests/ftrace: add loongarch support for kprobe args char tests

Add loongarch support for the recently added kprobe args tests.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: yuliao0214

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_char.tc

@@ -28,6 +28,9 @@ s390*)
 mips*)
   ARG1=%r4
 ;;
+loongarch*)
+  ARG1=%r4
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested