syscall: 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.
Change-Id: I5d9f780fba2710403fce89d3325e519f33ad9ae8
Reviewed-on: https://go-review.googlesource.com/c/go/+/618835 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>