runtime: fix race condition between GC and setGCPercent

If first GC runs concurrently with setGCPercent,
it can overwrite gcpercent value with default.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/10242047

src/pkg/runtime/mgc0.c

@@ -1974,7 +1974,10 @@ runtime·gc(int32 force)
 		return;
 
 	if(gcpercent == GcpercentUnknown) {	// first time through
+		runtime·lock(&runtime·mheap);
+		if(gcpercent == GcpercentUnknown)
 			gcpercent = readgogc();
+		runtime·unlock(&runtime·mheap);
 
 		p = runtime·getenv("GOGCTRACE");
 		if(p != nil)