CL 86295 changed MkdirAll to always pass a trailing path separator to
support extended-length paths on Windows.

However, when Stat is called on an existing file followed by trailing
slash, it will return a "not a directory" error, skipping the fast
path at the beginning of MkdirAll.

This change fixes MkdirAll to only pass the trailing path separator
where required on Windows, by using an OS-specific function fixRootDirectory.

Updates #23918

Change-Id: I23f84a20e65ccce556efa743d026d352b4812c34
Reviewed-on: https://go-review.googlesource.com/95255
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>