unix: add SYS___SYSCTL on darwin/arm

Add the same workaround as already present for 386 (CL 14151), amd64 (CL
14091) and arm64 (CL 14094) on darwin.

Fixes golang/go#22839

Change-Id: I7eee543df204410dd3328dccb7d81166f77e1e2f
Reviewed-on: https://go-review.googlesource.com/79155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_darwin_arm.go

@@ -60,3 +60,7 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno) // sic
+
+// SYS___SYSCTL is used by syscall_bsd.go for all BSDs, but in modern versions
+// of darwin/arm the syscall is called sysctl instead of __sysctl.
+const SYS___SYSCTL = SYS_SYSCTL