windows: add support for windows/arm

Updates golang/go#26148

Change-Id: I0425244bfb0c8f600a6f02d9d7b228e175a720a4
Reviewed-on: https://go-review.googlesource.com/127665
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/svc/sys_arm.s

@@ -0,0 +1,38 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+#include "textflag.h"
+
+// func servicemain(argc uint32, argv **uint16)
+TEXT ·servicemain(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4, R14], (R13)	// push {r4, lr}
+	MOVW    R13, R4
+	BIC     $0x7, R13		// alignment for ABI
+
+	MOVW	R0, ·sArgc(SB)
+	MOVW	R1, ·sArgv(SB)
+
+	MOVW	·sName(SB), R0
+	MOVW	·ctlHandlerExProc(SB), R1
+	MOVW	$0, R2
+	MOVW	·cRegisterServiceCtrlHandlerExW(SB), R3
+	BL      (R3)
+	CMP     $0, R0
+	BEQ     exit
+	MOVW	R0, ·ssHandle(SB)
+
+	MOVW	·goWaitsH(SB), R0
+	MOVW	·cSetEvent(SB), R1
+	BL      (R1)
+
+	MOVW	·cWaitsH(SB), R0
+	MOVW	$-1, R1
+	MOVW	·cWaitForSingleObject(SB), R2
+	BL      (R2)
+
+exit:
+	MOVW	R4, R13			// free extra stack space
+	MOVM.IA.W (R13), [R4, R15]	// pop {r4, pc}