Add remaining ordering strengths to atomic types

[Aatch]

We currently only have sequentially consistent (the strongest) and the acquire/release pair for ordering constraints. The intrinsics to support these have now been added, so the types should be extended to implement them.

[alchemy]

The atomic types defined in libstd/unstable/atomics.rs already have methods parametrized by memory ordering, that in turn call the appropriate intrinsic. Shouldn't this issue be closed?

[Aatch]

@alchemy yes, well spotted