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

usr.sbin/kldxref/Makefile

@@ -2,14 +2,11 @@
 PACKAGE=	runtime
 PROG=	kldxref
 MAN=	kldxref.8
-SRCS=	kldxref.c ef.c ef_obj.c
+SRCS=	kldxref.c ef.c ef_obj.c elf.c
+SRCS+=	ef_aarch64.c ef_amd64.c ef_i386.c ef_mips.c ef_powerpc.c ef_riscv.c
 
 WARNS?=	2
 
-.if exists(ef_${MACHINE_CPUARCH}.c)
-SRCS+=	ef_${MACHINE_CPUARCH}.c
-.else
-SRCS+=	ef_nop.c
-.endif
+LIBADD=	elf
 
 .include <bsd.prog.mk>