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
You mean `record_sp_limit`?    There's a number of uses which add a value other than RED_ZONE:
https://github.com/rust-lang/rust/blob/master/src/librustrt/stack.rs#L71 
https://github.com/rust-lang/rust/blob/master/src/libstd/rand/os.rs#L213
https://github.com/rust-lang/rust/blob/master/src/libstd/rand/os.rs#L217
I think it's better left alone.

Maybe call libnative variant `record_os_managed_stack_bounds`?