Add support for 64-bit Windows builds

Finally landed all the required fixes in LLVM master!

These 4 tests currently fail in win64 build, I suspect there's some ABI conformance bug:
run-pass/extern-pass-TwoU64s.rs
run-pass/extern-pass-empty.rs
run-pass/extern-return-TwoU64s.rs
run-pass/tcp-connect-timeouts.rs

Closes #1237
The `#[no_mangle]` shouldn't be necessary (nor the `rust_` prefix)