runtime: hide mheap from GC

It contains pointers to first blocks of lots of spans.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/11416046

src/pkg/runtime/malloc.goc

@@ -15,6 +15,8 @@ package runtime
 #include "race.h"
 #include "stack.h"
 
+// Mark mheap as 'no pointers', it does not contain interesting pointers but occupies ~45K.
+#pragma dataflag 16
 MHeap runtime·mheap;
 
 int32	runtime·checking;