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
This is to allow setting morestack stack limit from libnative without touching Windows's stack bounds TIB fields (gs:0x08 and gs:0x10).  Since libnative relies on os-managed stacks, it should not mess with these.  Currently it doesn't even do it correctly, because gs:0x10 is supposed to be the *commit* limit, not the reserve limit, as @klutzy had explained [here](http://github.com/rust-lang/rust/issues/3445#issuecomment-26114839).