Auto merge of rust-lang#31761 - Amanieu:volatile, r=alexcrichton

Tracking issue: rust-lang#31756
RFC: rust-lang/rfcs#1467

I've made these unstable for now. Should they be stabilized straight away since we've had plenty of experience with people using the unstable intrinsics?

1.6.0 release

1.5.0 release

1.4.0 release

1.3.0 release

1.2.0 release

1.1.0 release

1.0.0 release

1.0.0-alpha.2 release