kldxref: Reduce divergence between per-architecture files

Note that relbase is always 0 for DSOs so its omission for __KLD_SHARED
architectures was not a bug in practice.

Whilst here, also parenthesise the dest offset for where to avoid
transiently creating an out-of-bounds pointer, which is UB (though even
on CHERI architectures, where capability bounds compression can result
in that creating invalid capabilities that will trap on dereference,
optimisation will reassociate to the correct form in practice and thus
work just fine).

kldxref: Implement aarch64 R_AARCH64_AB64 relocation

Doing nothing seems to be sufficient but is strange, inconsistent with
other architectures, and not necessary when it's easy to support
properly.

kldxref: Cast rtype to int in ef_aarch64.c in warning

This is consistent with the other architecture-specific relocation
handlers.

Reported by: mjg
Sponsored by: DARPA

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

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

kldxref: improve style(9)

Address style issues including some previously raised in D13923.

- Use designated initializers for structs
- Always use bracketed return style
- No initialization in declarations
- Align function prototype names
- Remove old commented code/unused includes

Submitted by: Mitchell Horne <mhorne063@gmail.com>
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13943

Add support for relocating AArch64 modules to kldxref. This fixes an error
message where it fails to read the module as the unrelocated addresses
are zero.

Sponsored by: ABT Systems Ltd