kldxref: Make use of libelf to be a portable cross tool

This allows kldxref to operate on kernel objects from any
architecture, not just the native architecture. In particular, this
will permit generating linker.hints files as part of a cross-arch
release build.

• elf.c is a new file that includes various wrappers around libelf including routines to read ELF data structures such as program and section headers and ELF relocations into the "generic" forms described in <gelf.h>. This file also provides routines for converting a linker set into an array of addresses (GElf_Addr) as well as reading architecture-specific mod_* structures and converting them into "generic" Gmod_* forms where pointers are replaced with addresses.

• The various architecture-specific reloc handlers now use GElf_* types for most values (including GElf_Rel and GElf_Rela for relocation structures) and use routines from <sys/endian.h> to read and write target values. A new linker set matches reloc handlers to specific ELF (class, encoding, machine) tuples.

• The bits of kldxref.c that write out linker.hints now use the encoding (ELFDATA2[LM]SB) of the first file encountered in a directory to set the endianness of the output file. Input files with a different architecture in the same directory are skipped with a warning. In addition, the initial version record for the file must be deferred until the first record is finished since the architecture of the output file is not known until then.

• Various places that used 'sizeof(void *)' throughout now use 'elf_pointer_size()' to determine the size of a pointer in the target architecture.

Tested by: amd64 binary on both amd64 and i386 /boot/kernel
Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42966