runtime: signal handling support for plan9_arm

Plan 9 trap/signal handling differs on ARM from other architectures
because ARM has a link register.  Also trap message syntax varies
between different architectures (historical accident?).
Revised 7 March to clarify a comment.

Change-Id: Ib6485f82857a2f9a0d6b2c375cf0aaa230b83656
Reviewed-on: https://go-review.googlesource.com/18969
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/runtime/signal_plan9.go b/src/runtime/signal_plan9.go
index 19247f2..d3894c8 100644
--- a/src/runtime/signal_plan9.go
+++ b/src/runtime/signal_plan9.go
@@ -22,8 +22,8 @@
 	{_SigThrow, "sys: trap: invalid opcode"},
 
 	// We can recover from some memory errors in runtime·sigpanic.
-	{_SigPanic, "sys: trap: fault read addr"},  // SIGRFAULT
-	{_SigPanic, "sys: trap: fault write addr"}, // SIGWFAULT
+	{_SigPanic, "sys: trap: fault read"},  // SIGRFAULT
+	{_SigPanic, "sys: trap: fault write"}, // SIGWFAULT
 
 	// We can also recover from math errors.
 	{_SigPanic, "sys: trap: divide error"}, // SIGINTDIV