selftests/bpf: Enable inline bpf_kptr_xchg() test for RV64

Enable inline bpf_kptr_xchg() test for RV64, and the test have passed as
show below:

Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20240130124659.670321-3-pulehui@huaweicloud.com