syscall: implement Fchdir on Windows

syscall: implement Fchdir on Windows

This CL adds support for os.File.Chdir() on Windows by implementing
syscall.Fchdir, which is internally used by Chdir.

Windows does not provide a function that sets the working directory
using a file handle, so we have to fallback to retrieving the file
handle path and then use it in SetCurrentDirectory.

Change-Id: I2ae93575e50411e5a9426ea531541958d7c9e812
Reviewed-on: https://go-review.googlesource.com/c/go/+/480135
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>