selftests/ftrace: Add riscv support for kprobe arg tests

This is the riscv variant of commit 9855c462 ("selftests/ftrace:
Add ppc support for 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>