os.Chmod returns an error when passed a long path (>=260) characters on
Windows. CL 32451 fixed most file functions in os. This change applies the
same fix to os.Chmod.

Fixes #20829

Change-Id: I3270db8317ce6e06e6d77070a32a5df6ab2491e0
Reviewed-on: https://go-review.googlesource.com/47010Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>