Moved std::unstable::mutex to std::rt.
This is to cleanout the std::unstable module and use the appropriate
unstable attribute. This mutex module is **not** supposed to be
used outside of the compiler/std.

This change is according to #1457 and comment
https://github.com/mozilla/rust/issues/1457#issuecomment-40523194

[breaking-change]
auto merge of #14011 : TheHydroImpulse/rust/cleanout_unstable, r=alexcrichton

This is not a breaking change as mutex is re-exported in std::unstable
to make a transition easier. `std::mutex` is also marked as unstable.

This change is related to #1457 and comment
https://github.com/mozilla/rust/issues/1457#issuecomment-40523194