Added mingw specific build stubs.

R=rsc
CC=golang-dev
https://golang.org/cl/198071

src/pkg/debug/proc/proc_mingw.go

+// 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.
+
+package proc
+
+import "os"
+
+// Process tracing is not supported on MinGW yet.
+
+func Attach(pid int) (Process, os.Error) {
+	return nil, os.NewError("debug/proc not implemented on MinGW")
+}
+
+func ForkExec(argv0 string, argv []string, envv []string, dir string, fd []*os.File) (Process, os.Error) {
+	return Attach(0)
+}

src/pkg/debug/proc/regs_mingw_386.go

+// 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.
+
+package proc

src/pkg/debug/proc/regs_mingw_amd64.go

+// 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.
+
+package proc