runtime/debug: enable TestFreeOSMemory on all arches

TestFreeOSMemory was disabled on many arches because of issue #9993.
Since that's been fixed, enable the test everywhere.

Change-Id: I298c38c3e04128d9c8a1f558980939d5699bea03
Reviewed-on: https://go-review.googlesource.com/27403
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/debug/garbage_test.go

@@ -89,10 +89,6 @@ func TestReadGCStats(t *testing.T) {
 var big = make([]byte, 1<<20)
 
 func TestFreeOSMemory(t *testing.T) {
-	if runtime.GOARCH == "arm64" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOARCH == "mips64" || runtime.GOARCH == "mips64le" ||
-		runtime.GOOS == "nacl" {
-		t.Skip("issue 9993; scavenger temporarily disabled on systems with physical pages larger than logical pages")
-	}
 	var ms1, ms2 runtime.MemStats
 
 	if big == nil {