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
I explained this in the commit message.
Basically, it was not feasible to use gcc_personality_seh to both trigger unwind and to install the landing pad.  So these tasks had to be separated - the outer try/catch triggers the unwind, the inner one - stops it and returns the exception object.