Implement TotalEq and TotalOrd for Timespec

There's a test making sure that Ord works, so the order dependence
shouldn't be an issue
green: Don't return the red zone in stack_bounds()

This is mostly just an implementation detail, and anyone worried about the stack
bounds doesn't need to be bothered with the red zone because it's not usable
anyway.

Closes #12897
auto merge of #12906 : sfackler/rust/timespec-total, r=thestinger
auto merge of #12908 : alexcrichton/rust/issue-12897, r=thestinger

This is mostly just an implementation detail, and anyone worried about the stack
bounds doesn't need to be bothered with the red zone because it's not usable
anyway.

Closes #12897