unix: don't generate raw syscall wrapper for ClockGettime on darwin
unix: don't generate raw syscall wrapper for ClockGettime on darwin

Follow-up for CL 170299

CL 170297 erroneously introduced a raw syscall wrapper for ClockGettime
on darwin. Make sure it isn't generated by mistake again.

Change-Id: Ie9ecdc950e5ab6cc85937008cd140847cffac42f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/170301
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys