Mingw-w64 support

This patchset enables rustc to cross-build mingw-w64 outputs.
Tested on mingw + mingw-w64 (mingw-builds, win64/seh/win32-threads/gcc-4.8.1).

I also patched llvm to support Win64 stack unwinding.
https://github.com/klutzy/llvm/commit/ebe22bdbcebc4f8342c4c7c50e3b61fa6c68c1ad

I cross-built test/run-pass/smallest-hello-world.rs and confirmed it works.
However, I also found something went wrong if I don't have custom `#[start]` routine.