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>