runtime: print sigcode on signal crash

For #17496.

Change-Id: I671a59581c54d17bc272767eeb7b2742b54eca38
Reviewed-on: https://go-review.googlesource.com/32183
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/signal_sighandler.go

@@ -87,7 +87,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		print("Signal ", sig, "\n")
 	}
 
-	print("PC=", hex(c.sigpc()), " m=", _g_.m.id, "\n")
+	print("PC=", hex(c.sigpc()), " m=", _g_.m.id, " sigcode=", c.sigcode(), "\n")
 	if _g_.m.lockedg != nil && _g_.m.ncgo > 0 && gp == _g_.m.g0 {
 		print("signal arrived during cgo execution\n")
 		gp = _g_.m.lockedg