os: File.Seek does not work past EOF on NaCl

[dsnet]

Consider the following snippet:

f, err := ioutil.TempFile("", "")
if err != nil {
	log.Fatal(err)
}
// f.Truncate(1000)
if _, err := f.Seek(1000, io.SeekCurrent); err != nil {
	log.Fatal(err)
}

On NaCl, this current fails with:

seek /tmp/054003078: Invalid argument

If you uncomment the Truncate, then it works again. This is contrary to every other OS that Go supports, where you can seek past EOF and start writing and everything in-between is implicitly filled with zeros.

[gopherbot]

Change https://golang.org/cl/82535 mentions this issue: syscall: make Seek use SetFilePointerEx on Windows, allowing large seek offsets

[bradfitz]

Closing nacl bugs, as we're removing nacl (#30439)