readdir is disabled on linux for 386 and arm until syscall.seek is implemented there.
windows is hard, so leaving that for later.
File src/os/dir_other_go115.go can be deleted when we drop support for go 1.15.
Also adds TestReadNonDir, which was helpful while debugging.
File.Stat is left as a stub for now.
Tests are a bit stubbed down because os.ReadDir, os.Symlink, and t.TempDir are not yet (fully) implemented.
TODO: reimport tests from upstream as those materialize.
