Bug 1401647 - Add i686 target to win64-rust. r=ted

Add a cross-compilation copy of rust's standard library targeting
i686-pc-windows-msvc to the win64-rust toolchain package so it
can be used to build for win32 as well.

MozReview-Commit-ID: 3598VZrDjIH

--HG--
extra : rebase_source : f1b25a68a67ae7f9c505a42d17f29dbedf59a49d

taskcluster/ci/toolchain/windows.yml

@@ -114,6 +114,7 @@ win64-rust-1.22:
             '--channel', '1.22.1',
             '--host', 'x86_64-pc-windows-msvc',
             '--target', 'x86_64-pc-windows-msvc',
+            '--target', 'i686-pc-windows-msvc',
         ]
         toolchain-alias: win64-rust
         toolchain-artifact: public/build/rustc.tar.bz2