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.
* instruction decode/execute separated into class BytecodeMachine
* added randomx-tests project
* removed the use of non-portable __COUNTER__ macro
* removed the use of unsupported FENV_ACCESS pragma