runtime: consolidate arch-specific signal handlers on Plan 9

Change-Id: I4379418853c523fc9aaeb5d6f37bc96117841418
Reviewed-on: https://go-review.googlesource.com/1786
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>