Test both GNU targets on appveyor

appveyor.yml

@@ -12,14 +12,16 @@ environment:
   - TARGET: i686-pc-windows-msvc
     ARCH: x86
     VS: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
+  - TARGET: x86_64-pc-windows-gnu
+    MSYS_BITS: 64
   - TARGET: i686-pc-windows-gnu
-    ARCH: x86
+    MSYS_BITS: 32
 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"
   - if defined VS call "%VS%" %ARCH%
-  - 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