selftests/rseq: riscv: use rseq_get_abi() helper

Make the RISC-V rseq selftests compatible with glibc-2.35 by using the
rseq_get_abi() helper.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20220614154830.1367382-2-mjeanson@efficios.com