In wasm builds, #if out the impls for the 3 core kvvfs i/o routines which get overwritten from JS (and thus aren't used) to reduce dependencies on POSIX APIs. (tags: kvvfs-v2)

In wasm builds, #if out the impls for the 3 core kvvfs i/o routines which get overwritten from JS (and thus aren't used) to reduce dependencies on POSIX APIs. (tags: kvvfs-v2)