plan9: change Note to alias syscall.Note

This CL changes Note type from string to alias syscall.Note to make Note useful.

For golang/go#77076.

Change-Id: Ifb0292a6867ced95bb7e6c20877564cd527c1183
Reviewed-on: https://go-review.googlesource.com/c/sys/+/736480
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>