runtime: use correct MOV for plan9 brk_ ret value

Updates #11041

Change-Id: I78f8d48f00cfbb451e37c868cc472ef06ea0fd95
Reviewed-on: https://go-review.googlesource.com/27491
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_plan9_amd64.s

@@ -65,7 +65,7 @@ TEXT runtime·exits(SB),NOSPLIT,$0
 TEXT runtime·brk_(SB),NOSPLIT,$0
 	MOVQ	$24, BP
 	SYSCALL
-	MOVQ	AX, ret+8(FP)
+	MOVL	AX, ret+8(FP)
 	RET
 
 TEXT runtime·sleep(SB),NOSPLIT,$0