lib: posix: nanosleep

This change adds support for nanosleep(2) documented at
https://pubs.opengroup.org/onlinepubs/9699919799/functions/nanosleep.html



N.B: Currently, this provides no better resolution than
k_busy_wait()

Fixes #25554

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>