runtime: handle "sys: trap: divide error" note on Plan 9

Fixes #7286.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/61410044
diff --git a/src/pkg/runtime/signals_plan9.h b/src/pkg/runtime/signals_plan9.h
index f9bec65..b16ecaf 100644
--- a/src/pkg/runtime/signals_plan9.h
+++ b/src/pkg/runtime/signals_plan9.h
@@ -18,6 +18,7 @@
 	// exception.
 	P, "sys: trap: fault read addr",
 	P, "sys: trap: fault write addr",
+	P, "sys: trap: divide error",
 	T, "sys: trap:",
 
 	N, "sys: bad sys call",