os: implement readdir for darwin and linux

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.

src/os/file_go_116.go

@@ -8,19 +8,10 @@ import (
 )
 
 type (
-	DirEntry = fs.DirEntry
 	FileMode = fs.FileMode
 	FileInfo = fs.FileInfo
 )
 
-func (f *File) ReadDir(n int) ([]DirEntry, error) {
-	return nil, &PathError{"ReadDir", f.name, ErrNotImplemented}
-}
-
-func ReadDir(name string) ([]DirEntry, error) {
-	return nil, &PathError{"ReadDir", name, ErrNotImplemented}
-}
-
 // The followings are copied from Go 1.16 official implementation:
 // https://github.com/golang/go/blob/go1.16/src/os/file.go