NetBSD does not defined _XOPEN_UNIX so define it here

NetBSD does not have full support for _XOPEN_UNIX, but seems to have
enough to what is needed here. Probably core need a smarter test here
like:

 #defined(_POSIX_THREADS) && _POSIX_THREADS >= 200112L
Detect when pthread_getcpuclockid is available

If it is not available, don't implement unix_pthread_getcpuclockid and
protect its use in linux_ext module
backtrace is only available for X86_64 and LINUX_EXT