runtime: fix madvise for freebsd/amd64

Make use of carry clear condition instead of low or same.

R=minux.ma, jsing, dave
CC=golang-dev
https://golang.org/cl/6844080

src/pkg/runtime/sys_freebsd_amd64.s

@@ -190,8 +190,7 @@ TEXT runtime·madvise(SB),7,$0
 	MOVQ	24(SP), DX
 	MOVQ	$75, AX	// madvise
 	SYSCALL
-	CMPQ	AX, $0xfffffffffffff001
-	JLS	2(PC)
+	JCC	2(PC)
 	MOVL	$0xf1, 0xf1  // crash
 	RET