crypto/x509: skip arm64 tests limited by iOS

Just like darwin/arm.

Change-Id: Ib0438021bfe9eb105222b93e5bb375c282cc7b8c
Reviewed-on: https://go-review.googlesource.com/8822Reviewed-by: Minux Ma <minux@golang.org>

src/crypto/x509/root_darwin_test.go

@@ -10,8 +10,9 @@ import (
 )
 
 func TestSystemRoots(t *testing.T) {
-	if runtime.GOARCH == "arm" {
-		t.Skipf("skipping on %s/%s", runtime.GOOS, runtime.GOARCH)
+	switch runtime.GOARCH {
+	case "arm", "arm64":
+		t.Skipf("skipping on %s/%s, no system root", runtime.GOOS, runtime.GOARCH)
 	}
 
 	sysRoots := systemRootsPool()         // actual system roots

src/crypto/x509/x509_test.go

@@ -830,8 +830,9 @@ func TestImports(t *testing.T) {
 	case "android", "nacl":
 		t.Skipf("skipping on %s", runtime.GOOS)
 	case "darwin":
-		if runtime.GOARCH == "arm" {
-			t.Skipf("skipping on %s/%s", runtime.GOOS, runtime.GOARCH)
+		switch runtime.GOARCH {
+		case "arm", "arm64":
+			t.Skipf("skipping on %s/%s, cannot fork", runtime.GOOS, runtime.GOARCH)
 		}
 	}