syscall: don't truncate newly created files on Windows

syscall: don't truncate newly created files on Windows

There is no need for syscall.OpenFile to truncate newly created files.
Some special Windows files, like the NUL device, can't be
truncated, so we should avoid truncating unless it is really necessary.

Fixes #71752.

Change-Id: I8238048594f706f6a5281053d55cfe3dc898828d
Reviewed-on: https://go-review.googlesource.com/c/go/+/650276
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>