Bug 1543595 - Add missing aarch64 target to the rust 1.33 toolchain. r=froydnj

I must have written the rust 1.33 patch before I landed the
linux64-aarch64 patches, so when that landed, it lacked the aarch64
target. (it's still there on the rust 1.32 toolchain)

Differential Revision: https://phabricator.services.mozilla.com/D27035

--HG--
extra : moz-landing-system : lando

taskcluster/ci/toolchain/rust.yml

@@ -56,6 +56,7 @@ linux64-rust-1.33:
             '--host', 'x86_64-unknown-linux-gnu',
             '--target', 'x86_64-unknown-linux-gnu',
             '--target', 'i686-unknown-linux-gnu',
+            '--target', 'aarch64-unknown-linux-gnu',
         ]
         toolchain-alias: linux64-rust