zig.h: add casts for overflow arithmetic operations

This avoids the following error:

```
error: incompatible pointer types passing 'int64_t *' (aka 'long long *') to parameter of type 'long *'
    overflow = __builtin_saddl_overflow(lhs, rhs, res);
                                                  ^~~
```

My previous understanding was that this error would not occur because
prior to this line we check that int64_t is equivalent to long, like
this:

```c
```

However, it appears that this is still a warning in C if int64_t is
primarily aliased to `long long`, even though `long` and `long long` are
the same thing.

0 files changed, 0 insertions, 0 deletions