gc: limit helper threads based on ncpu

When ncpu < 2, work.nproc is always 1 which results in infinite helper
threads being created if gomaxprocs > 1 and MaxGcproc > 1. Avoid this
by using the same limits as imposed helpgc().

R=golang-dev, rsc, dvyukov
CC=golang-dev
https://golang.org/cl/5176044

src/pkg/runtime/mgc0.c

@@ -984,8 +984,7 @@ runtime·gc(int32 force)
 	// coordinate.  This lazy approach works out in practice:
 	// we don't mind if the first couple gc rounds don't have quite
 	// the maximum number of procs.
-	extra = work.nproc < runtime·gomaxprocs && work.nproc < MaxGcproc;
-
+	extra = work.nproc < runtime·gomaxprocs && work.nproc < runtime·ncpu && work.nproc < MaxGcproc;
 	runtime·starttheworld(extra);
 
 	// give the queued finalizers, if any, a chance to run