unix: re-add SYS___SYSCTL on darwin

CL 250437 removed all SYS_* consts on darwin as direct syscalls are no
longer supported on this platform. However, some external packages (e.g.
github.com/shirou/gopsutil) still seem to rely on SYS___SYSCTL being
defined to implement their own sysctl wrappers. Re-add SYS___SYSCTL to
avoid breaking these packages.

Change-Id: I1ef1c05ba47a4b89be61ee8919d1651b7531d9c7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/251738
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.go

diff --git a/unix/syscall_darwin.go b/unix/syscall_darwin.go
index 0cf31ac..e2a05ee 100644
--- a/unix/syscall_darwin.go
+++ b/unix/syscall_darwin.go
@@ -49,6 +49,11 @@
 	raw    RawSockaddrDatalink
 }
 
+// Some external packages rely on SYS___SYSCTL being defined to implement their
+// own sysctl wrappers. Provide it here, even though direct syscalls are no
+// longer supported on darwin.
+const SYS___SYSCTL = 202
+
 // Translate "kern.hostname" to []_C_int{0,1,2,3}.
 func nametomib(name string) (mib []_C_int, err error) {
 	const siz = unsafe.Sizeof(mib[0])