runtime: relax TestGcSys

This fixes occasional 64-bit failures.
Maybe it will fix the 32-bit failures too,
so re-enable on 32-bit for now.

R=golang-dev, bradfitz, r, dvyukov
CC=golang-dev
https://golang.org/cl/6218050

src/pkg/runtime/gc_test.go

@@ -10,11 +10,6 @@ import (
 )
 
 func TestGcSys(t *testing.T) {
-	if runtime.GOARCH != "amd64" {
-		// TODO(adg): remove this when precise gc is implemented
-		t.Logf("skipping on non-amd64 systems")
-		return
-	}
 	memstats := new(runtime.MemStats)
 	runtime.GC()
 	runtime.ReadMemStats(memstats)
@@ -31,6 +26,7 @@ func TestGcSys(t *testing.T) {
 	}
 
 	// Should only be using a few MB.
+	// We allocated 100 MB or (if not short) 1 GB.
 	runtime.ReadMemStats(memstats)
 	if sys > memstats.Sys {
 		sys = 0
@@ -38,7 +34,7 @@ func TestGcSys(t *testing.T) {
 		sys = memstats.Sys - sys
 	}
 	t.Logf("used %d extra bytes", sys)
-	if sys > 4<<20 {
+	if sys > 16<<20 {
 		t.Fatalf("using too much memory: %d bytes", sys)
 	}
 }