Fixed tests on OpenBSD

Since OpenBSD 6.0 W^X is enforced.
Added `RANDOMX_FLAG_SECURE` in tests and benchmarks.
Updated comment.

Excluded `cpu_set_t` since it is not defined on OpenBSD.

src/tests/affinity.cpp

@@ -65,7 +65,7 @@ set_thread_affinity(std::thread::native_handle_type thread,
             (thread_policy_t)&policy, 1);
 #elif defined(_WIN32) || defined(__CYGWIN__)
     rc = SetThreadAffinityMask(reinterpret_cast<HANDLE>(thread), 1ULL << cpuid) == 0 ? -2 : 0;
-#else
+#elif !defined(__OpenBSD__)
     cpu_set_t cs;
     CPU_ZERO(&cs);
     CPU_SET(cpuid, &cs);