unix: emit +build comments in generator scripts

Change-Id: I8ed64dcb1c1a2119ce6f15fa1be55fbe0b0c637e
Reviewed-on: https://go-review.googlesource.com/10087
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

unix/mksysnum_linux.pl

@@ -5,12 +5,19 @@
 
 use strict;
 
+if($ENV{'GOARCH'} eq "" || $ENV{'GOOS'} eq "") {
+	print STDERR "GOARCH or GOOS not defined in environment\n";
+	exit 1;
+}
+
 my $command = "mksysnum_linux.pl ". join(' ', @ARGV);
 
 print <<EOF;
 // $command
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
+// +build $ENV{'GOARCH'},$ENV{'GOOS'}
+
 package unix
 
 const(