cpu: enable TestARM64minimalFeatures on darwin/arm64

Go 1.16 renamed the iOS port from darwin/arm64 to ios/arm64 and
darwin/arm64 was repurposed for the macOS ARM64 port (see
https://golang.org/doc/go1.16#darwin).

Now that Go 1.16 is the oldest supported release, the ios tag can be
used exclusively to detect iOS. Thus, TestARM64minimalFeatures which
ought to run on darwin/arm64 can now be enabled on that platform.

For golang/go#45696

Change-Id: Ic510fbf27dc813832507446201501df58c9f6f31
Reviewed-on: https://go-review.googlesource.com/c/sys/+/353530
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_test.go

@@ -42,7 +42,7 @@ func TestAVX512HasAVX2AndAVX(t *testing.T) {
 }
 
 func TestARM64minimalFeatures(t *testing.T) {
-	if runtime.GOARCH != "arm64" || (runtime.GOOS == "darwin" || runtime.GOOS == "ios") {
+	if runtime.GOARCH != "arm64" || runtime.GOOS == "ios" {
 		return
 	}
 	if !cpu.ARM64.HasASIMD {