Test both GNU targets on windows

Author: alexcrichton

appveyor.yml

@@ -1,13 +1,16 @@
 environment:
   matrix:
+  - TARGET: x86_64-pc-windows-gnu
+    MSYS_BITS: 64
+  - TARGET: i686-pc-windows-gnu
+    MSYS_BITS: 32
   - TARGET: x86_64-pc-windows-msvc
   - TARGET: i686-pc-windows-msvc
-  - TARGET: i686-pc-windows-gnu
 install:
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-nightly-${env:TARGET}.exe"
   - rust-nightly-%TARGET%.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
-  - SET PATH=%PATH%;C:\Program Files (x86)\Rust\bin
-  - SET PATH=%PATH%;C:\MinGW\bin
+  - set PATH=%PATH%;C:\Program Files (x86)\Rust\bin
+  - if defined MSYS_BITS set PATH=%PATH%;C:\msys64\mingw%MSYS_BITS%\bin
   - rustc -V
   - cargo -V