sync/atomic: skip broken tests on freebsd/arm and netbsd/arm

Update #7338

The nil deref tests are currently failing on the *bsd/arm platforms. In an effort to avoid the build deteriorating further I would like to skip these tests on freebsd/arm and netbsd/arm.

LGTM=bradfitz, minux.ma
R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews
https://golang.org/cl/69870045

src/pkg/sync/atomic/atomic_test.go

@@ -1463,6 +1463,9 @@ func TestUnaligned64(t *testing.T) {
 }
 
 func TestNilDeref(t *testing.T) {
+	if p := runtime.GOOS + "/" + runtime.GOARCH; p == "freebsd/arm" || p == "netbsd/arm" {
+		t.Skipf("issue 7338: skipping test on %q", p)
+	}
 	funcs := [...]func(){
 		func() { CompareAndSwapInt32(nil, 0, 0) },
 		func() { CompareAndSwapInt64(nil, 0, 0) },