runtime: correct exception stack trace output

It is misleading when stack trace say:

signal arrived during cgo execution

but we are not in cgo call.

Change-Id: I627e2f2bdc7755074677f77f21befc070a101914
Reviewed-on: https://go-review.googlesource.com/9190Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/signal_windows.go

@@ -131,7 +131,9 @@ func lastcontinuehandler(info *exceptionrecord, r *context, gp *g) int32 {
 
 	print("PC=", hex(r.ip()), "\n")
 	if _g_.m.lockedg != nil && _g_.m.ncgo > 0 && gp == _g_.m.g0 {
-		print("signal arrived during cgo execution\n")
+		if iscgo {
+			print("signal arrived during external code execution\n")
+		}
 		gp = _g_.m.lockedg
 	}
 	print("\n")