syscall: prefer PATH environment variable in bootstrap scripts

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/9575043

src/pkg/syscall/mksyscall.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksyscall_windows.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksysctl_openbsd.pl

-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 # Copyright 2011 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style

src/pkg/syscall/mksysnum_darwin.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksysnum_freebsd.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksysnum_linux.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksysnum_netbsd.pl

-#!/usr/bin/perl
+#!/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.

src/pkg/syscall/mksysnum_openbsd.pl

-#!/usr/bin/perl
+#!/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.