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
If that's the case, this function needs to have some comments to justify its existence and explain what it's doing. Also, if the other function is *wrong*, why not just delete it? Surely it must be doing something?