Panicking tasks should abort process if not handled

RFC for issue opened here: https://github.com/rust-lang/rust/issues/19610

>It poisons mutable shared memory when it can contain arbitrary data.

The AtomicXXX structs do not appear to have poisoning, and they're a valid way of sharing arbitrary data, are they not? I'm going both by my example program's behaviour, as well as the source here: https://github.com/rust-lang/rust/blob/master/src/libcore/atomic.rs