unix: delete darwin/386 and darwin/arm code

This repo supports two Go releases, Go 1.15 and 1.16 (and tip).
The darwin/386 and darwin/arm ports are dropped in Go 1.15.
And these ports already do not build even with Go 1.14. Delete
them.

Change-Id: Ib15e7c35059967803a1d5f086b00fbfed53a9b33
Reviewed-on: https://go-review.googlesource.com/c/sys/+/316769
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_zos_test.go

@@ -168,7 +168,7 @@ func TestFcntlFlock(t *testing.T) {
 // "-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.GOARCH == "arm" || runtime.GOARCH == "arm64") {
+	if runtime.GOOS == "darwin" && runtime.GOARCH == "arm64" {
 		t.Skip("cannot exec subprocess on iOS, skipping test")
 	}
 
@@ -493,7 +493,7 @@ func TestGetwd(t *testing.T) {
 		dirs = []string{"/", "/system/bin"}
 	case "darwin":
 		switch runtime.GOARCH {
-		case "arm", "arm64":
+		case "arm64":
 			d1, err := ioutil.TempDir("", "d1")
 			if err != nil {
 				t.Fatalf("TempDir: %v", err)