unix: __sysctl is sysctl on darwin

While the other BSDs use __sysctl as the name, Darwin now uses sysctl,
without the leading underscores, and considers __sysctl to be "private".
Using __sysctl leads to App Store rejections, and Go's syscall package
already uses the proper syscall. So this commit changes Darwin's syscall
to use it too here, while reverting a recent commit that removed it all
together on arm and arm64.

This reverts CL 194097.

Fixes golang/go#35103
Updates golang/go#34133
Updates golang/go#35101

Change-Id: Ic72d5e7a435b99fe62c533b77b2c3790590f4c9e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/202837
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/darwin_test.go

@@ -89,7 +89,7 @@ var darwinTests = [...]darwinTest{
 	{"recvmsg", libc_recvmsg_trampoline},
 	{"sendmsg", libc_sendmsg_trampoline},
 	{"kevent", libc_kevent_trampoline},
-	{"__sysctl", libc___sysctl_trampoline},
+	{"sysctl", libc_sysctl_trampoline},
 	{"utimes", libc_utimes_trampoline},
 	{"futimes", libc_futimes_trampoline},
 	{"fcntl", libc_fcntl_trampoline},