unix: enable TestPassFD and TestPoll 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, TestPassFD and TestPoll which
ought to run on darwin/arm64 can now be enabled on that platform.

For golang/go#45696

Change-Id: I0b542af7a1b5ab194bf249d499c49e45bb55a2e1
Reviewed-on: https://go-review.googlesource.com/c/sys/+/353529
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>

unix/syscall_unix_test.go

diff --git a/unix/syscall_unix_test.go b/unix/syscall_unix_test.go
index 2f67283..4d66d1d 100644
--- a/unix/syscall_unix_test.go
+++ b/unix/syscall_unix_test.go
@@ -167,7 +167,7 @@
 // "-test.run=^TestPassFD$" and an environment variable used to signal
 // that the test should become the child process instead.
 func TestPassFD(t *testing.T) {
-	if (runtime.GOOS == "darwin" || runtime.GOOS == "ios") && runtime.GOARCH == "arm64" {
+	if runtime.GOOS == "ios" {
 		t.Skip("cannot exec subprocess on iOS, skipping test")
 	}
 
@@ -488,8 +488,7 @@
 }
 
 func TestPoll(t *testing.T) {
-	if runtime.GOOS == "android" ||
-		((runtime.GOOS == "darwin" || runtime.GOOS == "ios") && runtime.GOARCH == "arm64") {
+	if runtime.GOOS == "android" || runtime.GOOS == "ios" {
 		t.Skip("mkfifo syscall is not available on android and iOS, skipping test")
 	}