cmd/objdump: use cmd/internal/objfile

This removes a bunch of ugly duplicate code.
The end goal is to factor the disassembly code
into cmd/internal/objfile too, so that pprof can use it,
but one step at a time.

LGTM=r, iant
R=r, alex.brainman, iant
CC=golang-codereviews
https://golang.org/cl/149400043

src/cmd/internal/objfile/objfile.go

@@ -14,6 +14,8 @@ import (
 type rawFile interface {
 	symbols() (syms []Sym, err error)
 	pcln() (textStart uint64, symtab, pclntab []byte, err error)
+	text() (textStart uint64, text []byte, err error)
+	goarch() string
 }
 
 // A File is an opened executable file.
@@ -70,3 +72,11 @@ func (f *File) PCLineTable() (*gosym.Table, error) {
 	}
 	return gosym.NewTable(symtab, gosym.NewLineTable(pclntab, textStart))
 }
+
+func (f *File) Text() (uint64, []byte, error) {
+	return f.raw.text()
+}
+
+func (f *File) GOARCH() string {
+	return f.raw.goarch()
+}