Auto merge of rust-lang#3698 - folkertdev:sched-setaffinity, r=RalfJung

implement `libc::sched_setaffinity` on linux

fixes rust-lang/miri#2749

the implementation, like `libc::sched_getaffinity`, just always returns `EINVAL`, which kind of simulates a device with zero cpus. I believe the idea is that callers of this function always do it to optimize, so they are likely to gracefully recover from this function returning an error.

based on the libc crate, these functions are also available on android and freebsd (but not on macos or windows). So should the implementation of the `sched_*` functions just be copied to the android and freebsd shims?

Author: bors