alignof(expression) is GNU extension

Should be replaced by the type.

```
/Users/runner/work/s2geometry/s2geometry/src/s2/s2point_array.h:35:43: warning: 'alignof' applied to an expression is a GNU extension [-Wgnu-alignof-expression]
    ::operator delete(p, std::align_val_t(alignof(*p)));
                                          ^
```