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 <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/mksyscall.go

@@ -153,6 +153,11 @@ func main() {
 			}
 			funct, inps, outps, sysname := f[2], f[3], f[4], f[5]
 
+			// ClockGettime doesn't have a syscall number on Darwin, only generate libc wrappers.
+			if goos == "darwin" && !libc && funct == "ClockGettime" {
+				continue
+			}
+
 			// Split argument lists on comma.
 			in := parseParamList(inps)
 			out := parseParamList(outps)