go.sys/unix: use "use" in syscall interface

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144980043

windows/asm.s

@@ -0,0 +1,8 @@
+// Copyright 2014 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.
+
+#include "textflag.h"
+
+TEXT ·use(SB),NOSPLIT,$0
+	RET

windows/syscall.go

@@ -21,7 +21,10 @@
 // holds a value of type syscall.Errno.
 package windows
 
-import "syscall"
+import (
+	"syscall"
+	"unsafe"
+)
 
 // ByteSliceFromString returns a NUL-terminated slice of bytes
 // containing the text of s. If s contains a NUL byte at any
@@ -67,3 +70,8 @@ func (ts *Timespec) Nano() int64 {
 func (tv *Timeval) Nano() int64 {
 	return int64(tv.Sec)*1e9 + int64(tv.Usec)*1000
 }
+
+// use is a no-op, but the compiler cannot see that it is.
+// Calling use(p) ensures that p is kept live until that point.
+//go:noescape
+func use(p unsafe.Pointer)