[Object][ELF] Refactor ELFRelocationIterator into ELFEntityIterator. No functionality change.
[Object][ELF] Add program header iterator.
[objdump] Add --private-headers, -p.

This currently prints the ELF program headers.
[Object][ELF] Fix incorrect size of members for the 64 version of Elf_Phdr_Impl.