os: prevent infinite symlink loop of Stat on Windows

os: prevent infinite symlink loop of Stat on Windows

The Windows version of Stat calls Readlink iteratively until
reaching a non-symlink file.
If the given file is a circular symlink, It never stops.
This CL defines the maximum number of symlink loop count.
If the loop count will exceed that number, Stat will return error.

Fixes #16538

Change-Id: Ia9f3f2259a8d32801461c5041cc24a34f9f81009
Reviewed-on: https://go-review.googlesource.com/27580
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>