os, syscall: fix incorrect offset calculation in Readlink on windows

Current implementation of syscall.Readlink mistakenly calculates
the end offset of the PrintName field.
Also, there are some cases that the PrintName field is empty.
Instead, the CL uses SubstituteName with correct calculation.

Fixes #15978
Fixes #16145

Change-Id: If3257137141129ac1c552d003726d5b9c08bb754
Reviewed-on: https://go-review.googlesource.com/31118


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>