runtime: hide mheap from race detector

This significantly decreases amount of shadow memory
mapped by race detector.
I haven't tested on Windows, but on Linux it reduces
virtual memory size from 1351m to 330m for fmt.test.
Fixes #4379.

R=golang-dev, alex.brainman, iant
CC=golang-dev
https://golang.org/cl/6849057

src/pkg/runtime/race.c

@@ -34,9 +34,16 @@ static bool onstack(uintptr argp);
 void
 runtime·raceinit(void)
 {
+	uintptr sz;
+
 	m->racecall = true;
 	runtime∕race·Initialize();
-	runtime∕race·MapShadow(noptrdata, enoptrbss - noptrdata);
+	sz = (byte*)&runtime·mheap - noptrdata;
+	if(sz)
+		runtime∕race·MapShadow(noptrdata, sz);
+	sz = enoptrbss - (byte*)(&runtime·mheap+1);
+	if(sz)
+		runtime∕race·MapShadow(&runtime·mheap+1, sz);
 	m->racecall = false;
 }