runtime: implement runtime.osyield on FreeBSD 386, amd64

R=rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5689046

src/pkg/runtime/sys_freebsd_386.s

@@ -299,5 +299,9 @@ TEXT runtime·sysctl(SB),7,$28
 	MOVL	$0, AX
 	RET
 
+TEXT runtime·osyield(SB),7,$-4
+	MOVL	$331, AX		// sys_sched_yield
+	INT	$0x80
+	RET
 
 GLOBL runtime·tlsoffset(SB),$4

src/pkg/runtime/sys_freebsd_amd64.s

@@ -229,3 +229,7 @@ TEXT runtime·sysctl(SB),7,$0
 	MOVL	$0, AX
 	RET
 
+TEXT runtime·osyield(SB),7,$-4
+	MOVL	$331, AX		// sys_sched_yield
+	INT	$0x80
+	RET

src/pkg/runtime/thread_freebsd.c

@@ -154,9 +154,3 @@ runtime·sigpanic(void)
 	}
 	runtime·panicstring(runtime·sigtab[g->sig].name);
 }
-
-// TODO: fill this in properly.
-void
-runtime·osyield(void)
-{
-}