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