Teach readelf about some OpenBSD ELF program headers

- Add constants for OpenBSD wxneeded, bootdata and randomize to the
FreeBSD elf_common.h file. This is the file that gets used by the
elftoolchain library.
- Update readelf and elfdump utilities to decode these program headers
if they are encountered.

Note: FreeBSD has it's own version of elfdump(1), which will be updated
in a subsequent commit. I am adding it here anyway because this diff is
going to be submitted upstream.

Discussed with: emaste
Reviewed by: imp
MFC afer: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20548

M contrib/elftoolchain/elfdump/elfdump.c
M contrib/elftoolchain/readelf/readelf.c
M sys/sys/elf_common.h

Use the in-tree sys/elf_common.h to build libelftc.

This fixes build failures on older releases that lack various
definitions such as EM_AARCH64 (which was unfixed before this).

Revert all of the recent compatibility changes that worked around this
problem.

This uses the same method of using the in-tree header as lib/libelf,
lib/libdwarf and usr.bin/readelf.

Reviewed by: emaste
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D6734

elftoolchain: backwards compatability for EM_RISCV definition

It is not provided by sys/elf_common.h on older releases

Reported by: Jenkins

elftoolchain: backwards compatability for EM_IAMCU definition

It is not provided by sys/elf_common.h on older stable/10.

Rename ELFOSABI_SYSV to ELFOSABI_NONE to match current spec

Source: http://www.sco.com/developers/gabi/latest/ch4.eheader.html

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3731