runtime: handle abort note on Plan 9

Implement an abort note on Plan 9, as an
equivalent of the SIGABRT signal on other
operating systems.

Updates #11975.

Change-Id: I010c9b10f2fbd2471aacd1d073368d975a2f0592
Reviewed-on: https://go-review.googlesource.com/16300Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/signal_plan9.go

@@ -51,4 +51,7 @@ var sigtable = [...]sigTabT{
 
 	// Alarms can be handled if desired, otherwise they're ignored.
 	{_SigNotify, "alarm"},
+
+	// Aborts can be handled if desired, otherwise they cause a stack trace.
+	{_SigNotify + _SigThrow, "abort"},
 }