runtime: use SYSCALL instead of INT 0x80 in FreeBSD's runtime.osyield on amd64

R=mikioh.mikioh, rsc
CC=golang-dev
https://golang.org/cl/5690062

src/pkg/runtime/sys_freebsd_amd64.s

@@ -231,5 +231,5 @@ TEXT runtime·sysctl(SB),7,$0
 
 TEXT runtime·osyield(SB),7,$-4
 	MOVL	$331, AX		// sys_sched_yield
-	INT	$0x80
+	SYSCALL
 	RET