`zig cc -municode` does not define `UNICODE` macro

```
❯ cat test.c
int wmain() {}
#ifdef UNICODE
#error UNICODE is defined
#endif
❯ clang --target=x86_64-windows-gnu -municode test.c
test.c:3:2: error: UNICODE is defined
    3 | #error UNICODE is defined
      |  ^
1 error generated.
❯ zig cc --target=x86_64-windows-gnu -municode test.c
❯
```