syscall: don't run fcntl child process test on iOS

Fixes darwin-arm{,64} builds.
Child processes aren't allowed on iOS.

Change-Id: I9258ed4df757ec394ef6327dbda96f5b9705bcdd
Reviewed-on: https://go-review.googlesource.com/10142Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_unix_test.go

@@ -67,6 +67,9 @@ func _() {
 // Thus this test also verifies that the Flock_t structure can be
 // roundtripped with F_SETLK and F_GETLK.
 func TestFcntlFlock(t *testing.T) {
+	if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {
+		t.Skip("skipping; no child processes allowed on iOS")
+	}
 	flock := syscall.Flock_t{
 		Type:  syscall.F_WRLCK,
 		Start: 31415, Len: 271828, Whence: 1,