auto merge of #17349 : aturon/rust/rt-experimental, r=alexcrichton

The `std::rt` module was marked `unstable` [a while back](b6d4d11), and this change was not reverted when we moved to an `experimental` baseline for `std`.

Author: bors

src/libstd/lib.rs

@@ -263,7 +263,6 @@ pub mod fmt;
 
 // FIXME #7809: This shouldn't be pub, and it should be reexported under 'unstable'
 // but name resolution doesn't work without it being pub.
-#[unstable]
 pub mod rt;
 mod failure;