Enabling x86-64 JIT under Visual Studio compiler

Hello, Tim

Would adding cases for these assembly blocks that are in Visual Studio
syntax be enough to enable a working x86-64 JIT target under Windows
builds? I can't pretend to understand this codebase, so if there are
more roadblocks stopping 64-bit JIT on Windows targets I'd be
interested to hear...

The following things are needed for enabling win64 JIT in 2.2 release

1. Generalize defines in X86JITInfo.cpp
2. Provide win64 JIT compilation callback
3. Add new subtarget, so you can define such things as stack alignment,
etc
4. Implement a codegeneration for win64 calling convention

But please, don't invent a wheel - everything is done in LLVM
top-of-tree version (and newcoming 2.3 release).