windows: add LoadLibraryEx, add LazyDLL.System

Updates golang/go#14959

Change-Id: Ib91c359c3df919df0b30e584d38e56f79f3e3dc9
Reviewed-on: https://go-review.googlesource.com/21388
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

windows/registry/syscall.go

@@ -8,7 +8,7 @@ package registry
 
 import "syscall"
 
-//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go syscall.go
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -xsys -output zsyscall_windows.go syscall.go
 
 const (
 	_REG_OPTION_NON_VOLATILE = 0

windows/registry/zsyscall_windows.go

@@ -2,14 +2,17 @@
 
 package registry
 
-import "unsafe"
-import "syscall"
+import (
+	"golang.org/x/sys/windows"
+	"syscall"
+	"unsafe"
+)
 
 var _ unsafe.Pointer
 
 var (
-	modadvapi32 = syscall.NewLazyDLL("advapi32.dll")
-	modkernel32 = syscall.NewLazyDLL("kernel32.dll")
+	modadvapi32 = &windows.LazyDLL{Name: "advapi32.dll", System: true}
+	modkernel32 = &windows.LazyDLL{Name: "kernel32.dll", System: true}
 
 	procRegCreateKeyExW           = modadvapi32.NewProc("RegCreateKeyExW")
 	procRegDeleteKeyW             = modadvapi32.NewProc("RegDeleteKeyW")