windows/mkwinsyscall: use syscall.SyscallN instead of syscall.Syscall{6,9,12,15}

Replace syscall.Syscall6, Syscall9, Syscall12, and Syscall15 with
syscall.SyscallN for Go 1.18+. This simplifies system calls by allowing
the exact number of arguments needed, eliminating zero padding and
reducing potential errors.

Updated TestSyscallXGeneration to TestSyscallNGeneration to verify
correct SyscallN generation for different argument counts.

Change-Id: Iaf01c7bddd7ad6a80ee462879e382b0066f35b4d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/691715
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: David Chase <drchase@google.com>