runtime: fix 386 assembly for syscall.naclWrite

It was using the wrong offset and returned random values
making "runoutput" compiler tests crash.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/76250043

src/pkg/runtime/sys_nacl_386.s

@@ -35,7 +35,7 @@ TEXT syscall·naclWrite(SB), NOSPLIT, $12-16
 	MOVL SI, 4(SP)
 	MOVL DX, 8(SP)
 	CALL runtime·write(SB)
-	MOVL AX, ret+12(FP)
+	MOVL AX, ret+16(FP)
 	RET
 
 TEXT runtime·write(SB),NOSPLIT,$0