windows: implement Ftruncate using a single syscall on Windows

Ftruncate can be implemented on Windows using a single syscall. This
makes the implementation more efficient and less prone to races when
used in combination with other Seek calls.

Note that this is the x/sys counterpart for CL 618835.

Change-Id: Ie9be356bd953ccce85c0dd87a5dcc6ccf4fec464
Reviewed-on: https://go-review.googlesource.com/c/sys/+/621935
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>