cmd/addr2line, cmd/nm: factor object reading into cmd/internal/objfile

To do in another CL: make cmd/objdump use cmd/internal/objfile too.

There is a package placement decision in this CL:
cmd/internal/objfile instead of internal/objfile.
I chose to put internal under cmd to make clear (and enforce)
that no standard library packages should use this
(it's a bit dependency-heavy).

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/123910043

src/cmd/internal/objfile/objfile.go

@@ -0,0 +1,72 @@
+// Copyright 2014 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 objfile implements portable access to OS-specific executable files.
+package objfile
+
+import (
+	"debug/gosym"
+	"fmt"
+	"os"
+)
+
+type rawFile interface {
+	symbols() (syms []Sym, err error)
+	pcln() (textStart uint64, symtab, pclntab []byte, err error)
+}
+
+// A File is an opened executable file.
+type File struct {
+	r   *os.File
+	raw rawFile
+}
+
+// A Sym is a symbol defined in an executable file.
+type Sym struct {
+	Name string // symbol name
+	Addr uint64 // virtual address of symbol
+	Size int64  // size in bytes
+	Code rune   // nm code (T for text, D for data, and so on)
+	Type string // XXX?
+}
+
+var openers = []func(*os.File) (rawFile, error){
+	openElf,
+	openGoobj,
+	openMacho,
+	openPE,
+	openPlan9,
+}
+
+// Open opens the named file.
+// The caller must call f.Close when the file is no longer needed.
+func Open(name string) (*File, error) {
+	r, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	for _, try := range openers {
+		if raw, err := try(r); err == nil {
+			return &File{r, raw}, nil
+		}
+	}
+	r.Close()
+	return nil, fmt.Errorf("open %s: unrecognized object file", name)
+}
+
+func (f *File) Close() error {
+	return f.r.Close()
+}
+
+func (f *File) Symbols() ([]Sym, error) {
+	return f.raw.symbols()
+}
+
+func (f *File) PCLineTable() (*gosym.Table, error) {
+	textStart, symtab, pclntab, err := f.raw.pcln()
+	if err != nil {
+		return nil, err
+	}
+	return gosym.NewTable(symtab, gosym.NewLineTable(pclntab, textStart))
+}