Instead of calling Chmod directly on perm, stat the created file/dir to extract the
actual permission bits which can be different from perm due to umask.

Fixes #23120.

Change-Id: I3e70032451fc254bf48ce9627e98988f84af8d91
Reviewed-on: https://go-review.googlesource.com/84477
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>