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_freebsd.go

@@ -36,8 +36,6 @@ var (
 // INO64_FIRST from /usr/src/lib/libc/sys/compat-ino64.h
 const _ino64First = 1200031
 
-//sys	sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS___SYSCTL
-
 func supportsABI(ver uint32) bool {
 	osreldateOnce.Do(func() { osreldate, _ = SysctlUint32("kern.osreldate") })
 	return osreldate >= ver
@@ -203,6 +201,8 @@ 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
+
 func Uname(uname *Utsname) error {
 	mib := []_C_int{CTL_KERN, KERN_OSTYPE}
 	n := unsafe.Sizeof(uname.Sysname)