deprecate read/write eflags

[gnzlbg]

This deprecates __{read,write}eflags.

[gnzlbg]

Which rust version belongs exactly to the deprecation attribute? It might hit nightly in the next weeks, so it should be two stable versions from now (1.29) right?

[BurntSushi]

At a high level this LGTM, but I don't know enough about the particulars of how exactly things are deprecated to feel comfortable signing off on this.

cc @rust-lang-nursery/libs @alexcrichton

[alexcrichton]

FWIW these are covered by this block during verification which is why they passed through to stable without actually being defined by Intel.

I think it's a compelling enough argument to deprecate these intrinsics currently and we can always undeprecate if we really need to (or if some LLVM "bug" is fixed or something like that)

Thanks for the PR @gnzlbg!