runtime/debug: disable arm64 test for issue 9993

Like other arm64 platforms, darwin/arm64 has a different physical
page size to logical page size so it is running into issue 9993. I
hope it can be fixed for Go 1.5, but for now it is demonstrating the
same bug as the other skipped os+arch combinations.

Change-Id: Iedaf9afe56d6954bb4391b6e843d81742a75a00c
Reviewed-on: https://go-review.googlesource.com/8814Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/debug/garbage_test.go

@@ -88,8 +88,8 @@ func TestReadGCStats(t *testing.T) {
 var big = make([]byte, 1<<20)
 
 func TestFreeOSMemory(t *testing.T) {
-	if runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOOS == "nacl" {
-		t.Skip("issue 9993; scavenger temporarily disabled on systems with 64k pages")
+	if runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOOS == "nacl" || (runtime.GOOS == "darwin" && runtime.GOARCH == "arm64") {
+		t.Skip("issue 9993; scavenger temporarily disabled on systems with physical pages larger than logical pages")
 	}
 	var ms1, ms2 runtime.MemStats