Unused fields in struct stat

[sunfishcode]

The WASI libc stat implementation doesn't currently assign the following fields in struct stat: st_uid, st_gid, st_rdev, st_blocks, st_blksize.

The current WASI API doesn't currently provide the information needed to fill these fields in. So for each of these fields, we should decide whether to add something to WASI, leave the field as-is with a placeholder value (they're currently set to 0), or remove the field.

[sbc100]

It makes sense to me to leave them for possible future use. Since these modules can be thought of as emulating POSIX I can't see why we wouldn't one day want to extent to emulate users, groups, and block devices.