add stubs for `statvfs`, `chmod`, etc. (#463)

add stubs for `statvfs`, `chmod`, etc. (#463)

Per https://github.com/WebAssembly/wasi-sdk/issues/373, LLVM's libc++ no longer
allows us to enable `<fstream>` and `<filesystem>` separately -- it's both or
neither.  Consequently, we either need to patch libc++ to not use `statvfs`,
`chmod`, etc. or add stub functions for those features to `wasi-libc`.  Since
we're planning to eventually support those features with WASI Preview 2 and
beyond, it makes sense to do the latter.

Note that since libc++ uses `DT_SOCK`, I've added a definition for it -- even
though WASI Preview 1 does not define it.  No Preview 1 file will ever have that
type, so code that handles that type will never be reached, but defining it
allows us to avoid WASI-specific patches to libc++.

Related to `DT_SOCK`, I had to change the `S_IFIFO` value so it does not
conflict with `S_IFSOCK`, thereby avoiding ambiguity in `__wasilibc_iftodt`.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>