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/syscall_openbsd.go

@@ -18,8 +18,6 @@ import (
 	"unsafe"
 )
 
-//sys	sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS___SYSCTL
-
 // SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
 type SockaddrDatalink struct {
 	Len    uint8
@@ -180,6 +178,8 @@ func setattrlistTimes(path string, times []Timespec, flags int) error {
 
 //sys	ioctl(fd int, req uint, arg uintptr) (err error)
 
+//sys   sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS___SYSCTL
+
 //sys	ppoll(fds *PollFd, nfds int, timeout *Timespec, sigmask *Sigset_t) (n int, err error)
 
 func Ppoll(fds []PollFd, timeout *Timespec, sigmask *Sigset_t) (n int, err error) {