syscall: define common notes on Plan 9

syscall: define common notes on Plan 9

There is no signal list on Plan 9, since notes
are strings. However, some programs expect
signals to be defined in the syscall package.
Hence, we define a list of the most common notes.

Updates #11975.

Change-Id: I852e14fd98777c9595a406e04125be1cbebed0fb
Reviewed-on: https://go-review.googlesource.com/16301
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>