https://pubs.opengroup.org/onlinepubs/009695299/functions/pthread_once.html
POSIX does not specify that pthread_once() can return ENOSYS. That is a weird decision to make in libc. I'm not sure if there is much value in percolating the error upwards through tls_init() into a careful termination provides a huge amount of value because the error cannot be resolved without proper build & library support.
