runtime: fix deadlock detector false negative
The issue was that scvg is assigned *after* the scavenger goroutine is started,
so when the scavenger calls entersyscall() the g==scvg check can fail.
Fixes #5025.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7629045

src/pkg/runtime/runtime.h

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index d9afd5b..b027607 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -235,8 +235,9 @@
 	int8*	waitreason;	// if status==Gwaiting
 	G*	schedlink;
 	bool	ispanic;
-	bool	issystem;
-	int8	raceignore; // ignore race detection events
+	bool	issystem;	// do not output in stack dump
+	bool	isbackground;	// ignore in deadlock detector
+	int8	raceignore;	// ignore race detection events
 	M*	m;		// for debuggers, but offset not hard-coded
 	M*	lockedm;
 	int32	sig;