runtime: fix windows-amd64-2012 build

I forgot that in Go assembly, x+16(SP) is not the same as 16(SP).
The former is the virtual stack pointer (one word below FP on x86)
while the latter is the actual stack pointer.

Change-Id: Ibb7012bb97261949f5e1a0dc70869d9a6f50aa99
Reviewed-on: https://go-review.googlesource.com/c/go/+/176557
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_windows.go

@@ -135,7 +135,8 @@ var (
 // to start new os thread.
 func tstart_stdcall(newm *m)
 
-func ctrlhandler(_type uint32)
+// Called by OS using stdcall ABI.
+func ctrlhandler()
 
 type mOS struct {
 	waitsema uintptr // semaphore for parking on locks

src/runtime/sys_windows_amd64.s

@@ -211,7 +211,7 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0-0
 	JMP	sigtramp<>(SB)
 
 TEXT runtime·ctrlhandler(SB),NOSPLIT|NOFRAME,$8
-	MOVQ	CX, _type+16(SP)		// spill
+	MOVQ	CX, 16(SP)		// spill
 	MOVQ	$runtime·ctrlhandler1(SB), CX
 	MOVQ	CX, 0(SP)
 	CALL	runtime·externalthreadhandler(SB)