objdump: implement disassembly

There is some duplication here with cmd/nm.
There is a TODO to address that after 1.3 is out.

Update #7452

x86 disassembly works and is tested.

The arm disassembler does not exist yet
and is therefore not yet hooked up.

LGTM=crawshaw, iant
R=crawshaw, iant
CC=golang-codereviews
https://golang.org/cl/91360046