stage2: add support for integers in LLVM backend

Also adds support for simple operators, like add and subtract.
The intcast and bitcast instruction also have been implemented.

Linking with libc also works, so we can now generate working executables!

`zig build-exe example.zig -fLLVM -lc`:
```
fn add(a: i32, b: i32) i32 {
    return a + b;
}

export fn main() c_int {
    var a: i32 = -5;
    const x = add(a, 7);
    var y = add(2, 0);
    y -= x;
    return y;
}
```

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