windows: use go:linkname instead of assembly

This is a bit cleaner and makes it more explicit what's happening, along
with a documenting comment.

Change-Id: I30c92f8576c72b05ebdb4634c68023237bde3cbf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/199519
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

windows/asm_windows_386.s

@@ -1,13 +0,0 @@
-// Copyright 2009 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.
-
-//
-// System calls for 386, Windows are implemented in runtime/syscall_windows.goc
-//
-
-TEXT ·getprocaddress(SB), 7, $0-16
-	JMP	syscall·getprocaddress(SB)
-
-TEXT ·loadlibrary(SB), 7, $0-12
-	JMP	syscall·loadlibrary(SB)

windows/asm_windows_amd64.s

@@ -1,13 +0,0 @@
-// Copyright 2009 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.
-
-//
-// System calls for amd64, Windows are implemented in runtime/syscall_windows.goc
-//
-
-TEXT ·getprocaddress(SB), 7, $0-32
-	JMP	syscall·getprocaddress(SB)
-
-TEXT ·loadlibrary(SB), 7, $0-24
-	JMP	syscall·loadlibrary(SB)

windows/asm_windows_arm.s

@@ -1,11 +0,0 @@
-// 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.
-
-#include "textflag.h"
-
-TEXT ·getprocaddress(SB),NOSPLIT,$0
-	B	syscall·getprocaddress(SB)
-
-TEXT ·loadlibrary(SB),NOSPLIT,$0
-	B	syscall·loadlibrary(SB)

windows/dll_windows.go

@@ -11,6 +11,18 @@ import (
 	"unsafe"
 )
 
+// We need to use LoadLibrary and GetProcAddress from the Go runtime, because
+// the these symbols are loaded by the system linker and are required to
+// dynamically load additional symbols. Note that in the Go runtime, these
+// return syscall.Handle and syscall.Errno, but these are the same, in fact,
+// as windows.Handle and windows.Errno, and we intend to keep these the same.
+
+//go:linkname syscall_loadlibrary syscall.loadlibrary
+func syscall_loadlibrary(filename *uint16) (handle Handle, err Errno)
+
+//go:linkname syscall_getprocaddress syscall.getprocaddress
+func syscall_getprocaddress(handle Handle, procname *uint8) (proc uintptr, err Errno)
+
 // DLLError describes reasons for DLL load failures.
 type DLLError struct {
 	Err     error
@@ -20,10 +32,6 @@ type DLLError struct {
 
 func (e *DLLError) Error() string { return e.Msg }
 
-// Implemented in runtime/syscall_windows.goc; we provide jumps to them in our assembly file.
-func loadlibrary(filename *uint16) (handle uintptr, err syscall.Errno)
-func getprocaddress(handle uintptr, procname *uint8) (proc uintptr, err syscall.Errno)
-
 // A DLL implements access to a single DLL.
 type DLL struct {
 	Name   string
@@ -40,7 +48,7 @@ func LoadDLL(name string) (dll *DLL, err error) {
 	if err != nil {
 		return nil, err
 	}
-	h, e := loadlibrary(namep)
+	h, e := syscall_loadlibrary(namep)
 	if e != 0 {
 		return nil, &DLLError{
 			Err:     e,
@@ -50,7 +58,7 @@ func LoadDLL(name string) (dll *DLL, err error) {
 	}
 	d := &DLL{
 		Name:   name,
-		Handle: Handle(h),
+		Handle: h,
 	}
 	return d, nil
 }
@@ -71,7 +79,7 @@ func (d *DLL) FindProc(name string) (proc *Proc, err error) {
 	if err != nil {
 		return nil, err
 	}
-	a, e := getprocaddress(uintptr(d.Handle), namep)
+	a, e := syscall_getprocaddress(d.Handle, namep)
 	if e != 0 {
 		return nil, &DLLError{
 			Err:     e,