runtime: fix FreeBSD build

stab in the dark but plausible: the kernel does try to
return time zone information.
http://fxr.watson.org/fxr/source/kern/kern_time.c?v=FREEBSD8#L421

R=iant
CC=golang-dev
https://golang.org/cl/206053

src/pkg/runtime/freebsd/amd64/sys.s

@@ -61,6 +61,7 @@ TEXT	write(SB),7,$-8
 TEXT gettime(SB), 7, $32
 	MOVL	$116, AX
 	LEAQ	8(SP), DI
+	MOVQ	$0, SI
 	SYSCALL
 
 	MOVQ	8(SP), BX	// sec