Prevent exhaustive matching of Ordering to allow for future extension
The C++11 atomic memory model defines a `memory_order_consume` ordering which is generally equivalent to `memory_order_acquire` but can allow better code generation by avoiding memory barrier instructions. Most compilers (including LLVM) currently do not implement this ordering directly and instead treat it identically to `memory_order_acquire`, including adding a memory barrier instruction.
There is currently [work](http://open-std.org/Jtc1/sc22/wg21/docs/papers/2016/p0098r1.pdf) to support consume ordering in compilers, and it would be a shame if Rust did not support this. This PR therefore reserves a `__Nonexhaustive` variant in `Ordering` so that adding a new ordering is not a breaking change in the future.
This is a [breaking-change] since it disallows exhaustive matching on `Ordering`, however a search of all Rust code on Github shows that there is no code that does this. This makes sense since `Ordering` is typically only used as a parameter to an atomic operation.