os: stub out support for some more features

This is necessary for the following:

  - to make sure os/exec can be imported
  - to make sure internal/testenv can be imported

The internal/testenv package (which imports os/exec) is used by a lot of
tests. By adding support for it, more tests can be run.

This commit adds a bunch of new packages that now pass all tests.

src/os/file_other.go

@@ -48,3 +48,7 @@ func (f stdioFileHandle) Close() error {
 
 //go:linkname putchar runtime.putchar
 func putchar(c byte)
+
+func Pipe() (r *File, w *File, err error) {
+	return nil, nil, ErrNotImplemented
+}