Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove ELF dynamic tag definitions that aren't used. Ideally,
those that are used should come from <machine/elf.h>.

s/R_IA64_/R_IA_64_/g as per the ia64 psABI.

This file was repocopied from src/sys/boot/efi/arch/ia64.

o Support the REL32LSB relocation. It's in the ELF file from which
we construct the EFI image. It doesn't seem to actually end up
in the EFI image, AFAICT.
o Replace .quad, .long and .short with data8, data4 and data2 resp.
The former are gnuisms.
o Redefine _start_plabel as a data16 with @iplt(_start) as its
value. This is the preferred way to create user PLT entries.

Remove _putchar, _puts and _puthex. These functions are unused.

Approved by: re (blanket)

Improve self-relocation:
o We don't expect the PLT relocations to follow the .rela section
anymore. We still assume that PLT relocations are long formed,
o Document register usage,
o Improve ILP,
o Fix the FPTR relocation by creating unique OPDs per function.
Comparing functions is valid now,
o The IPLT relocation naturally handles the addend. Deal with it.
We ignore the addend for FPTR relocations for now. It's not at
all clear what it means anyway.

Fix ABI misinterpretation:
o For Elf_Rela relocations, the addend is explicit and should not
be loaded from the memory address we're relocating. Only do that
for Elf_Rel relocations (ie the short form).
o DIR64LSB is not the same as REL64LSB. DIR64LSB applies to a
symbol (S+A), whereas REL64LSB applies to the base address (BD+A),

o Don't include sys/cdefs.h

Try to get the self-relocator to work with IPLTLSB relocations. Doesn't
work right though - I can't figure out why.

Remove a 'return' statement which I put in while I was trying to debug
the startup code.

First approximation of an ia64 EFI loader. Not functional.