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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Björn Töpel <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/kptr_xchg_inline.c

···
 	unsigned int cnt;
 	int err;
 
-#if !(defined(__x86_64__) || defined(__aarch64__))
+#if !(defined(__x86_64__) || defined(__aarch64__) || \
+      (defined(__riscv) && __riscv_xlen == 64))
 	test__skip();
 	return;
 #endif