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
Thinking about this, could you just update libnative to call `record_stack_limit` as opposed to `record_stack_bounds`. You can move the `RED_ZONE` addition into `record_stack_limit` as well