runtime: Make gc_test test extra allocated space, not total space.

Testing total space fails for gccgo when not using split
stacks, because then each goroutine has a large stack, and so
the total memory usage is large.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5487068

src/pkg/runtime/gc_test.go

@@ -6,16 +6,24 @@ import (
 )
 
 func TestGcSys(t *testing.T) {
+	runtime.GC()
+	runtime.UpdateMemStats()
+	sys := runtime.MemStats.Sys
+
 	for i := 0; i < 1000000; i++ {
 		workthegc()
 	}
 
 	// Should only be using a few MB.
 	runtime.UpdateMemStats()
-	sys := runtime.MemStats.Sys
-	t.Logf("using %d MB", sys>>20)
-	if sys > 10e6 {
-		t.Fatalf("using too much memory: %d MB", sys>>20)
+	if sys > runtime.MemStats.Sys {
+		sys = 0
+	} else {
+		sys = runtime.MemStats.Sys - sys
+	}
+	t.Logf("used %d extra bytes", sys)
+	if sys > 2<<20 {
+		t.Fatalf("using too much memory: %d bytes", sys)
 	}
 }