WasmFS: Support non-private mmap

The `_mmap_js` implementation in syscalls.cpp has a TODO:

```
// TODO: handle non-private mmaps. Those can be optimized in interesting ways
// like avoiding an allocation and a copy as we do below (whereas a
// private mmap is always a copy into a new, private region not shared
// with anything else).
```

It also asserts that the flags contain `MAP_PRIVATE`. We don't need to fully address the optimizations in the TODO yet, but we do need to do better than asserting on `MAP_SHARED`.