stage2: add LLVM codegen windows support to the self-hosted compiler

The following example generates a valid `main.exe`:
`zig build-exe main.zig -fLLVM -target x86_64-windows-gnu --subsystem console`

```
export fn wWinMainCRTStartup() noreturn {
    foo();
    exit();
}

fn foo() void {}

fn exit() noreturn {
    unreachable;
}
```
b059bb84b8Timon Kruiper committed on 12/28/2020, 8:20:46 PMparent0714171
1 file changedLine totals unavailable