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

unix/mksysnum_darwin.pl

@@ -0,0 +1,32 @@
+#!/usr/bin/env perl
+# 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.
+#
+# Generate system call table for Darwin from sys/syscall.h
+
+use strict;
+
+my $command = "mksysnum_darwin.pl " . join(' ', @ARGV);
+
+print <<EOF;
+// $command
+// MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
+
+package syscall
+
+const (
+EOF
+
+while(<>){
+	if(/^#define\s+SYS_(\w+)\s+([0-9]+)/){
+		my $name = $1;
+		my $num = $2;
+		$name =~ y/a-z/A-Z/;
+		print "	SYS_$name = $num;"
+	}
+}
+
+print <<EOF;
+)
+EOF