Teach readelf about some OpenBSD ELF program headers

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