runtime: fix gccheckmark mode and enable by default

It was testing the mark bits on what roots pointed at,
but not the remainder of the live heap, because in
CL 2991 I accidentally inverted this check during
refactoring.

The next CL will turn it back off by default again,
but I want one run on the builders with the full
checkmark checks.

Change-Id: Ic166458cea25c0a56e5387fc527cb166ff2e5ada
Reviewed-on: https://go-review.googlesource.com/9824
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgcmark.go

@@ -673,7 +673,7 @@ func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork
 
 			throw("checkmark found unmarked object")
 		}
-		if !hbits.isCheckmarked() {
+		if hbits.isCheckmarked() {
 			return
 		}
 		hbits.setCheckmarked()

src/runtime/runtime1.go

@@ -336,6 +336,7 @@ var dbgvars = []dbgVar{
 }
 
 func parsedebugvars() {
+	debug.gccheckmark=1
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""
 		i := index(p, ",")