Auto merge of #1028 - est31:master, r=alexcrichton

Re-add aarch64 stuff removed by previous PR

Previous PR #1023 has removed stuff from mod.rs and added it to some submodules, but
missed the aarch64 submodule.

This caused a build failure for rust on aarch64, see https://github.com/rust-lang/rust/pull/51864.

This copies the stuff that that commit added to the x86_64.rs submodule
and puts it into aarch64.rs.

It's been tested in the rust-lang/rust PR above: https://travis-ci.org/rust-lang/rust/builds/398750336