runtime: fix windows/amd64 exception handler

R=golang-dev, rsc
CC=golang-dev, hectorchu
https://golang.org/cl/5797077

src/pkg/runtime/sys_windows_amd64.s

@@ -137,7 +137,7 @@ TEXT runtime·sigtramp(SB),7,$0
 	get_tls(CX)
 
 	// check that m exists
-	MOVQ	m(BX), AX
+	MOVQ	m(CX), AX
 	CMPQ	AX, $0
 	JNE	2(PC)
 	CALL	runtime·badsignal(SB)