Add rumprun CI and get tests passing

[alexcrichton]

I was curious to see how we could get rumprun running over Thanksgiving, and it turns out it ended up working! I ended up having to fix a ton of definitions as well, and unfortunately I haven't been able to verify OpenBSD or Bitrig, so they may have broke as a result of this as well.

cc @gandro

[gandro]

This is awesome. Looking at the code, I think we could use target_os = "netbsd" instead of feature = "rumprun", since they are basically the same from libc's perspective. I hope to provide NetBSD CI as well at some point. I was working on it this weekend, but I ran out of time.

[alexcrichton]

Yeah I've been meaning to get a NetBSD VM up and running to verify that these definitions are the same on NetBSD (there's a bunch of different ones), but I'll take your word for it for now!