windows: change Signal to alias syscall.Signal

This CL changes Signal type from int to alias syscall.Signal
to make Signal useful. In particular, you can pass windows.SIGINT
to signal.Notify to produce behaviour similar to os.Interrupt and
syscall.SIGINT.

For golang/go#77076.

Change-Id: Ie9243c30063e777f9898eba6d649c248a7f48fc5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/735920
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>