Implement sched_getaffinity

[sunfishcode]

sched_getaffinity is used in the num_cpus crate, as well as the available_concurrency feature in Rust nightly. This will help with Mustang, as reported here.

The first step is to create a struct which wraps a cpu_set_t and provides these operations as methods.

Once that's defined, sched_getaffinity should be similar to other system calls.