Fix naming of windows installer
auto merge of #17135 : brson/rust/wininst, r=alexcrichton

This builds on https://github.com/rust-lang/rust/pull/17109, putting the target triple into the installer name so that we can have both 32-bit and 64-bit.

The resulting installers will be called `rust-0.12.0-pre-x86_64-w64-mingw32.exe`, etc.