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

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

@@ -13,7 +13,8 @@ void test_kptr_xchg_inline(void)
 	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