Add TID support to sched_getaffinity/sched_setaffinity

[RalfJung]

Our sched_getaffinity/sched_setaffinity shims currently only support a pid of 0, indicating the current thread. However, now that we have gettid, we should support using a TID in sched_getaffinity/sched_setaffinity as well. We probably want some helper functions to convert between a LinuxTid and a ThreadId, to ensure this is consistent.

Note that gettid is Linux-specific, while we support sched_getaffinity/sched_setaffinity also on FreeBSD. This new support for non-0 pid should remain Linux-specific for now; gettid does not seem to be exposed by the libc on FreeBSD.