runtime: disable gcMarkRootCheck debugging check during STW

gcMarkRootCheck takes ~10ns per goroutine. This is just a debugging
check, so disable it (plus, if something is going to go wrong, it's
more likely to go wrong during concurrent mark).

We may be able to re-enable this later, or move it to after we've
started the world again. (But not for 1.6.x.)

For 1.6.x.

Fixes #14419.

name / 95%ile-time/markTerm          old          new  delta
500kIdleGs-12                24.0ms ± 0%  18.9ms ± 6%  -21.46%  (p=0.000 n=15+20)

Change-Id: Idb2a2b1771449de772c159ef95920d6df1090666
Reviewed-on: https://go-review.googlesource.com/20148Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/22044Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -1560,7 +1560,8 @@ func gcMark(start_time int64) {
 	gcDrain(&gcw, gcDrainBlock)
 	gcw.dispose()
 
-	gcMarkRootCheck()
+	// TODO: Re-enable once this is cheap.
+	//gcMarkRootCheck()
 	if work.full != 0 {
 		throw("work.full != 0")
 	}