runtime: perform gcMarkRootCheck during STW in checkmark mode

gcMarkRootCheck is too expensive to do during mark termination.
However, since it's a useful check and it complements checkmark mode
nicely, enable it during mark termination is checkmark is enabled.

Change-Id: Icd9039e85e6e9d22747454441b50f1cdd1412202
Reviewed-on: https://go-review.googlesource.com/20663Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -1554,8 +1554,11 @@ func gcMark(start_time int64) {
 	gcDrain(gcw, gcDrainBlock)
 	gcw.dispose()
 
-	// TODO: Re-enable once this is cheap.
-	//gcMarkRootCheck()
+	if debug.gccheckmark > 0 {
+		// This is expensive when there's a large number of
+		// Gs, so only do it if checkmark is also enabled.
+		gcMarkRootCheck()
+	}
 	if work.full != 0 {
 		throw("work.full != 0")
 	}