runtime: Ignore stack sizes when deciding when to GC.

    
    This restores https://golang.org/cl/6081043 which was lost accidentally
    when updating a new version of libgo in https://golang.org/cl/22440043 .
    
    Reviewed-on: https://go-review.googlesource.com/14569

From-SVN: r227784

gcc/go/gofrontend/MERGE

-ae60deadd72b3b29df98cee61deed68f251f0122
+23392287e9a26956977987fe95f337c5be4d6417
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/mgc0.c

@@ -2280,7 +2280,7 @@ gc(struct gc_args *args)
 	heap0 = mstats.next_gc*100/(gcpercent+100);
 	// conservatively set next_gc to high value assuming that everything is live
 	// concurrent/lazy sweep will reduce this number while discovering new garbage
-	mstats.next_gc = mstats.heap_alloc+mstats.heap_alloc*gcpercent/100;
+	mstats.next_gc = mstats.heap_alloc+(mstats.heap_alloc-runtime_stacks_sys)*gcpercent/100;
 
 	t4 = runtime_nanotime();
 	mstats.last_gc = runtime_unixnanotime();  // must be Unix time to make sense to user