go.sys: copy files from syscall package to go.sys/{plan9,windows,unix}

This CL copies to each package of go.sys the files from syscall it will need.
Different directories have different files, but these:
        mkall.sh
        str.go
        syscall.go
        mksyscall.pl
        race.go
        race0.go
        syscall_test.go
are copied to all three.
No changes yet, these are just copies. They are not ready to use yet:
package names are wrong, for starters. But this clean copy will make
it easier to follow the changes as the packages are enabled.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126960043

windows/syscall_windows_test.go

@@ -0,0 +1,72 @@
+// Copyright 2012 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.
+
+package syscall_test
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+)
+
+func TestWin32finddata(t *testing.T) {
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	path := filepath.Join(dir, "long_name.and_extension")
+	f, err := os.Create(path)
+	if err != nil {
+		t.Fatalf("failed to create %v: %v", path, err)
+	}
+	f.Close()
+
+	type X struct {
+		fd  syscall.Win32finddata
+		got byte
+		pad [10]byte // to protect ourselves
+
+	}
+	var want byte = 2 // it is unlikely to have this character in the filename
+	x := X{got: want}
+
+	pathp, _ := syscall.UTF16PtrFromString(path)
+	h, err := syscall.FindFirstFile(pathp, &(x.fd))
+	if err != nil {
+		t.Fatalf("FindFirstFile failed: %v", err)
+	}
+	err = syscall.FindClose(h)
+	if err != nil {
+		t.Fatalf("FindClose failed: %v", err)
+	}
+
+	if x.got != want {
+		t.Fatalf("memory corruption: want=%d got=%d", want, x.got)
+	}
+}
+
+func abort(funcname string, err error) {
+	panic(funcname + " failed: " + err.Error())
+}
+
+func ExampleLoadLibrary() {
+	h, err := syscall.LoadLibrary("kernel32.dll")
+	if err != nil {
+		abort("LoadLibrary", err)
+	}
+	defer syscall.FreeLibrary(h)
+	proc, err := syscall.GetProcAddress(h, "GetVersion")
+	if err != nil {
+		abort("GetProcAddress", err)
+	}
+	r, _, _ := syscall.Syscall(uintptr(proc), 0, 0, 0, 0)
+	major := byte(r)
+	minor := uint8(r >> 8)
+	build := uint16(r >> 16)
+	print("windows version ", major, ".", minor, " (Build ", build, ")\n")
+}