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